Packaging it as a standalone binary for different platforms seems like a good plan. I have no idea how hard this might be to do though, sorry. But you can always propose it in one of KDE's chat channels or mailing lists and see if it piques anyone else's interest or they feel like helping you do it!

You're very welcome! I'm happy you're enjoying it.

from what I understand, you can only have them both installed (or none), not one or the other

You can have only one installed, and Fedora KDE ships this way. If your distro doesn't offer that, it seems like a distro packaging issue.

make it easier to change file associations for common file types

That's in Plasma 6. :)

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

One of the most visible ones for me is that most common multimonitor workflows Just Work™ in the Wayland session now. There are still edge cases, but we've put a huge amount of effort into this.