... anything that would allow me to be cross-platform, like .net, ...
I've never used anything but my own code to be cross-platform. Like you, I believe those "cross-platform, like .net" frameworks are an abomination and balloon out the effort.
And in terms of UI, not a single line of code can and should be shared among platforms. (With the exception of macOS and iOS, there's some overlap if done right. But less than one would think.)
Well, I think the artwork can be shared
. Some/most of the human factors architecture (not code) can be as well. That human factors stuff is time consuming and costly.
But what do I know, I've only been doing this for 20 years…
Yeah, my relative lack of experience at 40 years working on almost every variety of Unix there is, VM/SP, VAX/VMS, macOS both pre- and post- NeXT acquisition, all manner of X Window System frameworks, clearly pales next to your experience. I have managed to avoid DOS, for the most part, thank the stars.
Remember my last job was Mac/Win file system so I know about cross-platform file APIs. Another job I had was Mac/Win network code, so I know about that, too. And there's nothing more different than the network stacks on VM/SP and VAX/VMS.
Not saying it's easy. If you're know you're doing it from Day One it's very manageable. If, on Day One, you think "We'll target Windows (or anything else) First" then you've lost.
There's a person on another forum who would want your software as that person is not enamored of the instant kill switch in roon 2.0. Help that person out and get writing!