Wayland and X11 can exist in parallel. I have multiple desktop environments with some defaulting to Wayland and some still using X11. For my casual machine, I use XFCE most of the time ( X11 ) but have been toying around with the new COSMIC ( Wayland ). I switch back and forth.
So, the X11 on your system will not hold you back when you move to Wayland. Of course, at some point the old stuff is just cruft. So you do have to do a bit of janatorial from time to time.
I use a rolling release.
The AUR is the best thing about Arch. yay -Syu and everything is updated. Painless.
I tend to use binary packages to avoid long compiles. If an update includes something that is going to take a while, I often exclude that package from the update. After everything else is updated, I can run it again to get the last package or two. They can just run in the background while I do other stuff. If it is a program I am going to use right away, I may put off the update of that package until I am done my session. This is pretty common with JetBrains updates for example.
I do not have a single Flatpak.