Improving room acoustics can get super expensive

Another option is to try much lower reverb settings or have no reverb at all.  Yes, reflections are undesirable for critical listening (as in doing music production to resell), but if you had an acoustic piano in the space, it would have the same reflections and you probably wouldn't be bothered much.

I would recommend either a small room reverb mixed to -12db to give a little bit of space to the sound or pianoteq totally dry.  See how either option works.  Odds are that you can use the room to your advantage rather than redesign it for critical listening.

Also, those recommendations will vary significantly based on what speakers you are using, since the bass traps are only really needed for frequencies below 40hz, and most 8" monitors don't even go below 50-60hz, and a dedicated subwoofer usually has to be 10-12" before it will solidly reproduce those lowest frequencies (15-35hz).  So if you chose to treat the room, you could probably skip out on the bass traps.  If you have a "flutter echo" (which can do using a simple clap test and hear if you get a distinct repetition of the full sound one or more times), that's probably more important to correct and can be done with the regular wall covering material and/or a small cloud.  However, since you're not trying to make an acoustic reproduction environment, I would guess that you'll be just fine embracing the acoustics of the space rather than layering heavy synthetic reverb over the natural reverb of the space.

The only time I recall having a real problem with an acoustic in a real space was in an auditorium foyer where we had a flutter echo that repeated about 5 times and it was like playing a piano inside a bell.  That would be a situation where the natural acoustic would be unacceptable, but it was a 10ft x 30ft x 25ft space made entirely of concrete and the weirdest shapes of the walls--whereas a glass box like you describe should be rather comfortable.

If a room sounds nice/natural to talk in, then it will be a good room to play the piano.

Furthering Beto-Music's comments, you can buy monitors like the IK Multimedia MTM (which I have just purchased but not received) which come with a mic and have built in DSP to analyse your room and correct the sound at the listener's location (or wherever you place the mic).   I believe there are other monitors that can do the same.   People claim it works quite well.

Headphones: Much better and much cheaper ... but
(1) you have to endure having them on your ears for long hours,
(2) accepting less natural sound management in space when you turn your head ,
(3) give up the conviviality of a piece of music shared live ...
Possibly, for a certain form of conviviality in this period of generalized covid confinement, it is theoretically possible to imagine playing live with headphones by connecting Pianoteq to Jamulus, (hearing possible all over the planet in almost real time) but with large sound quality problems in the presence of too many listeners and with a "weak" public feedback perceptible by the pianist .

With Zoom configured in stereo mode, better quality for more people (up to 100 free) but with more latency and provided you know how to invite for a musical session.
For these tools, one thing that Pianoteq standalone lacks is the possibility of being directly recognized as a stereo microphone. Currently it is necessary to put between Pianoteq and the broadcasting tool (Jamulus, Zoom or other ...) a virtual card like Voice meeter, VB cable ... unfortunately sometimes difficult to adjust and often introducing latency ... An output direct from Pianoteq simulating a virtual microphone could sometimes be practical.

Bruno

Acoustic cancellation is theoretically an option, but I'm afraid that would simply take away as much or more direct sound while trying to tame the reflections.  Since you can't amplify a room mode or null to cure it (if you amplify cancellation it stays the same volume--rather like multiplying by zero), I don't know that there would be any improvement from adding more acoustic issues to the space to cure the problems already there.  Headphones are an option, but I don't think they're the experience that the OP really wants.

It's also important to remember that it's easy to start over-treating a room, since the best critical listening spaces have some level of ambience that strategically absorbs most acoustic momentum while leaving enough resonance and reflection to still feel "natural."  Often absorption isn't balanced with enough diffusion and the highs and mids (which are easier to tame with cheap materials) are very "dead" and the bass still continues to "ring" too long.  Without a small amount of ambience, we'd spend life mixing audio in an anechoic chamber, which would definitely be a thing of nightmares!

Starting with as dry a signal as possible and letting the space add the reverb would probably still be the simplest and most life-like solution.

You can also use Room EQ Wizard (REW) to take some measurements of the space and see exactly what the room physics are doing, and we could offer better solutions based on copies of those readouts.

Wanted to add more to extoll the use of bookcases for damping walls:

1. One advantages of using tall bookcases with books is that the books can be removed or moved easily, unlike panels that may require the
drilling of new holes or the attaching of more velcro to walls.
2. If more damping is needed, you can always attach acoustic panels inside the bookcase, lining the back. You probably will not. Books make very good dampers.
3. Bookcases can hold other things--monitors can go on top. Rack units can be set on the shelves. Mic adapters can attach a boom arm to a bookcase. The boom arm can be removed between sessions, or it can be swiveled into the bookcase. (For a tall boom mic, with the mic hanging down, this can be an ideal arrangement--the big conventional mic stands are large and take up a lot of floor space, and must be counter-balanced against the weight of the mic. No such problems if the boom arm is attached to something heavy, such as a bookcase.
4.They hold books. (!) If you do not use bookcases for sound damping, you must store your books elsewhere, taking up space, and making them harder to retrieve when you need them.

Of course, for a pro studio with clients coming in, this would look strange. But for a private studio, simple bookcases merit attention.

Hello guys,

not sure if I really should go so crazy with the bookcases. Have made a small mockup, how I could place stuff.
But is it appealing?
https://streamable.com/et6su9

In reality, the current room looks quite a bit different, but just so you get an idea. The radiators and windows are a pain, also the door opens inwards.

Compare to real room: https://streamable.com/n6i41h

Ideas?