Hi there, I’m not entirely sure if that’s exactly what you are thinking about, but anyway.
I recently implemented a feature to the mGBA snap (at https://github.com/popey/mgba-snap/pull/5). What this does is that it will explicitly set the GTK theming to use the qt5-style-plugins. This shouldn’t affect the theming when running natively on KDE at all, on GTK, it will default to said Qt5 theming.
If you want to take a look at the screenshots: This shows the Qt based application running on “stock” Ubuntu 19.10.
I think the key part here is that you shouldn’t use gtk3 for QT_QPA_PLATFORMTHEME, but gtk2 as I did in my example. From what I understood while researching this before I was able to update the mGBA snap, that’s because somehow the qt5-style-plugins are meant to be used that way - maybe you could give this a try?
If you want a really deep integration in both DEs, then I think that’s something that should be adressed in the application itself - I don’t think that’s specific to being a snap…