I keep coming back to this and not finding / choosing a solution. It sadly feels like people who are doing this are just going with Electron now. I'm mostly coding in Go now, so I seem to be looking for a similar solution to you.

This page has a lot of cross-platform GUI toolkits, but it focuses on C++:

https://philippegroarke.com/posts/2018/c++_ui_solutions/

I've been drawn to wxWidgets (actual native controls on each platform) or JUCE (most of the cross-platform commercial Windows/Mac/Linux audio effects I've bought are made in JUCE). But I've not had a chance to give either a proper try.

Years ago I used to all my cross-platform work with Xojo, a kind of cross-platform Visual Basic. That actually worked well for me, but then you're writing code in the Xojo Basic language, and it had some odd file formats for projects (not just raw text source files like C++ / Go would give me). Once LLMs hit I felt I probably needed to move on from Xojo to something less proprietary: https://www.xojo.com/