The thing here is that affinity designer, Illustrator or photoshop are not originally made for UI design. Sketch is made for this which which gives you an advantage over the the other tools here.

In general I recommend to try out tools often to find the right tools for the job. So with OS: You have to try it out and then decide which OS supports your workflow better.

Personally I ended up using Win10 / MacOS / Ubuntu altogether on a regular basis, because there is no OS which fits it all.

And so the zeitgeist is: In the future there will be more tools which support all platforms trough the power of the web. You can give figma a shot to see a good example for such a multi-platform tool.