Thanks for the pointers @mwestphal!
We have not had problems with line rendering as in the first issue. In two of our widgets, where we do volume rendering, we’ve in fact run with anti aliasing off. This was out of necessity since we were rendering volumes sitting right next to each other, and the MSAA of the two volumes would result in a thin white streak (so similar reason for Paraview not being able to use MSAA for its parallel rendering). Since then we’ve moved to using FXAA instead (post-processing approach), which does not look as nice, but is better than nothing.
For the second issue, we do not have a fullscreen mode in our app, and even so, I’m not sure this would be applicable to us since we use FXAA (?).
And the third link is actually an MR of my own, adding support to the native widget for fractional device pixel ratios 
All in all, I now think our best course of action is to report the remaining issue we have on macOS, and move back to using the native widget in all contexts. I think that’s easier than doing it selectively only on macOS.
I forgot to say here one detail about the macOS issue we have: It seems to be non-deterministic, sometimes it happens and sometimes not, so I suspect it’s some race somewhere internally (VTK, Qt, driver, …). I can try to make a test case same as I did with the Windows issue.