GTK 4 and libadwaita are completely separate thing. You can use gtk 4 without using libadwaita. Libadwaita is for the apps that likes to integrate closely with Gnome HIG. Elementary OS has granite. You can develop any app using gtk4. take a look at Zrythm DAW. It uses gtk 3 but looks completely different than most of the normal gtk 3 apps. Whenever Zrythm ports to gtk 4, it can look similar.
I primarily use the linux releases of Reaper and Renoise. Reaper for live recording, fine tuning etc. Renoise as a sketchpad for quick loops and chops.
I also use MIXXX (think serato) for scratches etc.
I previously used and financially support Ardour, which I still think warrants support, and believe it will turn into something very robust in the not-too-distant future. Another one to look out for in future is ZRythm. Still in the early stages of development, but very promising, and FOSS.
I also like the approach of [Zrythm](https://www.zrythm.org/en/index.html) where you could download the source and compile it yourself, or pay a few dollars and get a pre-compiled binary.
You can use snap store from canonical. Flathub is working on paid apps support, something called flatpak-transaction. Or you can host your binary in your own website. Actually, you can sell a software even if it is opensource. Check out Armorpaint (Similar to Substance Painter) and Zrythm (DAW). They are both opensource software but you need to buy the binary version from their websites. You can also build those softwares from source as an alternative.
I use Ubuntu Studio as my primary system. My secondary runs vanilla Ubuntu.
For DJing I'm using Mixxx with a Denon MC7000. Mixxx can absolutely be used with controller hardware. It's just not very intuitive or friendly. However, you can hook up it to basically anything that talks MIDI. At one point I was using a combination of a Hercules DJControl Compact and an M-Audio Oxygen8 keyboard to do what I needed.
For producing I use LMMS to sketch out ideas. I like the beat editor and how it feels like old school FL. For anything beyond that I use Reaper. It's paid, but it's very reasonably priced, has an unlimited "try" period, and works very well. Pair it up with Carla and you can use nearly VST, as long as it doesn't have aggressive authentication schemes like izotope software, for example.
As I mentioned in response to another comment, https://www.zrythm.org/en/index.html looks interesting and I'd definitely like to give it a try. Ardour is another option, but I've found it very.... difficult to ramp up on and I've had some stability problems with it.
I switched my primary system out a long time ago and after repeated BSODs and other assorted MS bullshit, nuked my secondary system and set it up for Ubuntu this past weekend.
LMMS is good for roughly sketching out ideas, but it doesn't replace a DAW.
I saw this recently and haven't had a chance to play with it, but it looks interesting. https://www.zrythm.org/en/index.html