My understanding was that there are significant changes between QT5 and QT6, at least that is the reason I was told by people that used it in our company. There was a warning to never upgrade to QT6. I am just going by the observation, could be wrong though.
Re Riverside forums: PyQt isn't Qt and isn't even the official Python bindings for Qt, but a third-party product. Only a small part of Qt is GPL, the vast majority is LGPL (always with commercial as the alternative). (Indeed, given that PyQt is GPL + commercial third-party, it seems to show that dual-licensing components that link to Qt is indeed possible)
Only if you're unwilling to be using GPL in the first place, which is clearly the desired effect. (If somewhat unfair for small companies, that have good reasons for not using a libre license and can't afford a multi-licensing expert.)
Regardless, the licensing is a total nightmare, you can see it in Riverside's own forums: https://pyqt.riverbankcomputing.narkive.com/hqREGR1W/a-nasty...