Good to hear Ivar.
Saw this thread a while back, loved that others had such good advice. Since I posted elsewhere today too about some basic mastering ideas, and today is a day I put aside for posting here and other Pianoteq fun for me.. so here's some thoughts in case any of it helps you @civilizedchaos, or others. Hope the text wall, without too much editing/formatting isn't too much of a bother to grind through.
I enjoy the idea of auto or online mastering but haven't found an ideal one, nor one I'd recommend for anything other than some modern genres, and as you notice it can leave you without much fine control. It might get more interesting as AI can understand more instruction (like we may be able to tell the app specific things, down to sections, or per bar etc.).
If using a mastering engineer, offer a detailed running sheet with instructions. You can say obvious things, like "Prefer staying on mix / please discuss any expressive change to EQ / subtle preferred to genre specific hype of any aspect".. IDK, I think the ones I've known and worked with would appreciate clients stating those things. Not because they're smug and think they know better but because, they work magic for clients mostly. Many really DO know what your potential listers DO want to hear!, as much as we might not like to think that For example, for much of my experimental music, every detail in the mix to master is the way I want it.. but NOBODY could say what genre any of it is for - every track quite different, except for some themed projects. I'm not personally a very 'stick to one thing' kind of musician which has been a strength and a difficult aspect for me. But.. a good mastering engineer has made much nicer results from the releases I've been on, than anything I would have done myself (although I love my own ways, I know they are more for me, than for other subjective listeners a lot of the time).
For example, many of us making our own music might think we know exactly how it should sound on a final file. But.. maybe we don't really know as well as we'd like to think we do. We might be close (closer than I am to being within a genre or with an obviously nearly marketable sound).
Maybe this is the most common reason, when people finally think "Maybe I should master my own music so it sounds even better than this mix I just finished", that..
they get to a kind of dead space.. they look for some auto solution, seek Youtube files (and get a heap of hyper blathering sales pitches for all kinds of products and "look at me and my fun video" presenters - some great, some OK painfully not good to follow along with).
So, if thinking "I'm trying one more time with a mastering engineer", if you can't sit in on the session, definitely a whole lot of data with time-stamps etc. to really dig in, to say things like "I know it's obvious to punch the bass up for this, or to give a almost double track vibe on the chorus but I'm consciously avoiding those genre specific cues".. keep it honest, obvious and unless your engineer is just starting out, they should understand - esp. with a phone conversation or zoom call etc.. of sitting in on session if poss.
Most will be more likely to get the best possible results that way IMHO.. although it will be a cost and some other skills to know how to direct things, without being a client from hell. Some engineers like to make all the decisions "It's why they book me" kind of a deal. You live with what you get back - and it's likely to be high art, IF that person really is with a high reputational status. Plenty are not - and choosing a good ME is not easy.. so there's that going against it too.
What the others recommended above is good advice - and by experimenting with good tools, you'll soon find your own ideal output
There are many other tools too which I like and probably use more for a mastering line up.
I'm not using Ozone as much these days but anyone's intended finished result will kind of decide which tools to use, after using a few and find what you like about each.
Rarely am I sticking to one setup, but often shuffling different plugins while finding a preferred outcome for specific pieces (mostly like you say instrumental - experimental modern etc. as well as solo piano).
Some mastering worthy plugins by Brainworx are wonderful too, like their "bx_masterdesk" or "Shadow Hills Class A Mastering Compressor" or "SPL IRON" - listed here from IMO most potentially gentle to more generally risky. You can push these hard, or choose very subtle settings beyond defaults - and IMO any of these 3 could make anyone but the most fastidious very happy, for any kind of music (I mention these, because you mention you didn't like your mastering engineer going into too much alteration of the main mix - these kinds of tools can help you avoid you overdoing the mastering process with iZotope gadgets all over the sonic results, similar to your critique of your mastering engineer, aha!).
If a mix is good (already EQd the way it needs etc.), then perhaps just one of those 3 make a great final touch.
To me, it's also perhaps a bit more like using hardware of old (like a now vintage Sadie or others - in case anyone remembers those desks and like me, enjoys working that way to this day).
Worth saying, it also helps me to consider mastering as not a place for too too much work..
preferring to more completely finish fine details in the mix, then take that file into the mastering stage.
Really do believe separating workflows in that way helps make better creative decisions in mix too.
At least that's always been my way of seeing it. In a studio, you record, mix and output a final mix.. and in early days, we'd later make a master tape - more or less just a bunch of us finesse-ing faders and going over and over until we got a final woohoo 'perfect' result for the day. That tape = the master tape. Later, digital formats etc.. but similar deal.
With later times, a record company might own the master from the studio (until artists recoup maybe - always different deals). That master may be a digital or physical or collection of such from which derivatives can be said to have been made authentically. Often 're-mastered' should only apply to one who uses a genuine master as source for the re-master. Some 'master collections' lmay include entire tape libraries accrued during album recording - but in general the concept continues into the digital age, where arguably, any MP3 of something can be good enough to re-mix or re-master for some other video/streaming platform.. so the concept has indeed drifted to mean some more nebulous and 'doesn't matter so much these days' kind of things.
For most of us making music from home studios, our idea of 'master' can be quite creative.
But generally.. I see it all as this for most of us..
Record tracks.
Mix them (adding plugins where desired)
Master = output a bunch of files, or 1. Aiming each for exact streaming services, or just go for -14 LUFS.
Esp. for creative modern music, none of that might matter enormously - and you could be very far from sounding like anyone else, and it could be your thing to have a certain sound per album or song..
but for someone aiming for top of the charts in any genre, then there will be lots of more specific tenets and procedures you might like to learn, try out and/or follow.
iZotope gives lots of options which are useful, but IDK, I'd rather do all that minutia in the mix, rather than do so much in mastering.
If the mix is not very complete or has lots of issues built in, then iZotope is a fine tool for digging into in all kinds of ways (I don't think of it as a mastering only thing though I guess - mostly using it on some things before mastering using other tools).
Metering.. you might like to output files to -14 LUFS, or another measures for specific streaming services and there are many tools to measure it. I've maybe mentioned a few here before - but lately I've been really happy with "ADPTR Streamliner". Also in my fav DAW "Studio One", you can output files to many streaming and particular LUFS requirements, in batch. No need to singularly make file after file.. just a few clicks and checkboxes and voila - a heap of files ready for different platforms.
In the mix, I like to get a fairly close to -14 LUFS nowadays, before taking that file into mastering (others may like up or down, doing things differently). For me, that way, I'm not tasking the mastering gear with too much crazy headroom changing work.. focus can be more on all the subtle alterations which can make a mix become more enjoyable. Things you might not think about when mixing (aiming for vibe).
Everyone will have their own ideas and methods - but hopeful that some of this above gives you some things to toss around.
Good luck! Hope you enjoy the process and attain outcomes to your liking.
Pianoteq Studio Bundle (Pro plus all instruments) - Kawai MP11 digital piano - Yamaha HS8 monitors