Before I make suggestions, I would like to turn the question back to you and get some more information.
What do you think defines good user experience? Are there any existing distributions or environments that come close to what •you* think represents good user experience?
Do you want to contribute financial resources, marketing resources, development resources or ...?
I would try to think outside of the GNOME/KDE boxes if you can, but it is perfectly acceptable to think either of those are good representatives.
Are there any existing solution out there that you think come close or any blue sky projects that really excite you?
Rather than just say "You should support Project X", I would like to know more about your thoughts and opinions.
I think desktop environments do think about user experience, and try to have a cohesive experience. Bad experience is easier to identify. Many programs, even major ones such as GIMP, libreoffice, or thunderbird don’t (in my opinion) prioritise user experience over features. As for blue sky thinking, while there are some interesting projects, I think linux just needs some consistency and a good baseline of UX to allow non technical consumers to easily use it. In many regards Apple gets this right on macOS (although maybe less so with recent changes, and with reduced options for power users), and many third party apps for Mac think carefully about their design. For the record, some unusual UX I like include concepts app (iOS), scrivener, iA writer, ‘bum too’ desktop environment, and pathways.nice.org.uk (niche)
I would like to financially donate as I lack time and necessary level of skill to directly help with projects.
I think the ship has sailed long time ago. Today, when users hear Linux on desktop, it means you have to use the console to have things done, which isn't a bad thing but makes software makes expect Linux users to sort out issues when on Mac/Windows if you fail to deliver working software it means your product is not existing in main distribution channel.
Take most common software package, Firefox, here is how to install it on Ubuntu: https://support.mozilla.org/en-US/kb/install-firefox-linux.
You have to install a flatpack first. As a user I want to go the the software website, click install and that's it. To make Linux usable on a desktop, it has to appeal. Currently the appeal is it's free and some of the distros don't have a spyware/ads integrated, which is awesome.
I can't see any ways of Linux and its 600 distros becoming flagship user experience, freeBSD on the other hand might have a chance.
The biggest complaint is in the area of applications.
Especially from long time users of applications that don't run on GNU/Linux, who whine until they have a pixel-for-pixel, event-for-event exact equivalent.
"You can't be serious about about GNU Replacement Program; why it doesn't even have anything like the Floozney plugin. How can anyone use such a piece of garbage. And the UI looks like it's from about 2013."
Also, be selfish. When you're complaining about something, and the devs say "My grandmother can use it", don't just accept that their grandmother's user experience is more important than your own!