What’s the best or recommended way to test out Plasma 6 RC2?
Neon Testing in a VM (or on bare metal if you’re adventurous). Arch with the kde-unstable repo is good too, but that also includes a snapshot of the unreleased Qt 6.7 which introduces more bugs.
What has been the hardest problem to solve moving to Qt6?
Personally I’d have to say the large number of API and behavior changes in QtQuick that Qt 6 has brought. We use QtQuick very heavily throughout KDE, so this has required a lot of mandatory porting work, more than in our QtWidgets-based software. And there have even been changes between Qt 6.5, 6.6, and 6.7, so it’s still a bit of a moving target