Raspberri + Touchscreen?

Yes, it is possible and in fact the Pianoteq Demo and configuration for touchscreen is included as part of the ZynthianOS install for use on Raspberry Pi's with some additional hardware, including touchscreens.  You can buy an official Zynthian hardware kit.  If you're more adventurous, you can build your own using parts of your choice.  Info on all of this is on Zynthian website and wiki: 
website: https://zynthian.org/#
wiki:  https://wiki.zynthian.org/index.php/Zynthian_Wiki_Home

I use the Raspberry/Ipad solution as described in that thread (but with some enhancements such as typing on the piano keyboard) but I also sometimes use as an alternative a Mac Mini controlled by the Ipad which is very easy to set up. So just the very neat little Mac Mini sitting on the piano with the Ipad on the music rest. No mouse or keyboard or screen for the Mac.
I happened to have an Ipad and a Mac Mini so the cost for setting it up was zero for me. Maybe there are better solutions like the one you're considering with an NUC if you're starting from scratch.
Just one point - an Ipad can be a bit small for displaying music pdf's.

You're right, Zynthian solution is probably not for you.  It is a Linux distribution, and definitely helps if you're willing to get your hands a little dirty.

Remember that Pianoteq will at some point be coming out for iOS, presumably for both iPad and iPhones.  They have said before that it will be "soon".  Who knows what that means, but I suspect that would be ideal solution for you, maybe you want to wait a bit longer for it.

The processing power in even the basic iPad is greater than that in many if not most sub $1k Windows laptops, and multiple times greater than what's in a Raspberry Pi.  Same goes for iPhones, very fast, way more than enough power for Pianoteq.  And the M1 iPads are essentially equivalent to the M1 Macs, which are way overkill for merely running Pianoteq. 

I still have an iPad Pro from 2017, which is way faster than my much newer Raspberry Pi 4, would have no problem with Pianoteq.

I've found the MeLE Quieter3C to be a nice alternative to the Raspberry Pi for running Pianoteq.  It's much faster than a Raspberry Pi4 and you don't have to worry about what parts to order or how to assemble them.  It comes preassembled in a tiny fanless case.  It even comes with Windows preloaded and it's Intel based so if you wanted you could just run Pianoteq for Windows and plug in a tiny touchscreen monitor.

https://www.amazon.com/gp/product/B0B765VF84

I'm running Ubuntu Mate on it, which is a Linux distribution a lot like Raspberry Pi OS but which can run on a wide variety of hardware.  I updated the pianoteq-pi script so that it works with Ubuntu Mate and also fixed some problems the script had with the latest versions of Raspberry Pi OS, and made some other improvements.  That means you can set everything up without any knowledge of Linux at all, although it's nice to know your way around a little bit in case anything goes wrong.  After running the script all you need to do is turn the thing on and Pianoteq starts running automatically.

You can access the UI using VNC on an iPad or on any other Windows/Mac/Linux computer on your network.  You can also use any kind of device that sends MIDI to control most features in Pianoteq, and there are some different web-based remote control apps for Pianoteq available as well (one modification I made to the pianoteq-pi script is to enable the option in Pianoteq to handle those web-based UI commands).  That said, if you really fiddle with the Pianoteq UI all the time the best thing is just to plug in a touch screen monitor (or a regular monitor and a mouse or trackpad).  Personally I tend to pick one good piano sound and never touch the Pianoteq UI after that so I don't need any kind of monitor or remote control at all under normal circumstances.

Here's a picture where I have the Pianoteq computer and a small headphone amp velcro-ed to the back of my keyboard.

Yeah I saw that.  They're having enough trouble just making Pi4's with the chip shortages.  Normally a Pi4 would be much cheaper than the Quieter3C but with the prices a lot of places are charging for Pi4's they're not that much difference in price.

I get a performance index in the mid 50's in Pianoteq (without overclocking), versus upper 20's if I recall on an overclocked Pi4.  I can also run at 48kHz internal sample rate and with a buffer size of 64.  I think I was limited to 24kHz internal sample rate on the Pi4 and had to use a larger buffer.

I can select them on my pi4 but it's very easy to overload the CPU with those settings.  On the MeLE I can pound away on all the keys all day and it doesn't come anywhere near overloading the CPU.

Yes, I've run these tests before with no overloading on the Raspberry Pi 400.

Here's an example:
https://forum.modartt.com/viewtopic.php...30#p973230

I think I've done this at 48 kHz as well:
https://forum.modartt.com/viewtopic.php...33#p973233

I use this literally every day with no problem.

I have always suspected my PreSonus Studio 24c has something to do with it, but I don't have an understanding of how that fits into the performance equation.

Yes, the MeLE looks good to me.