With the first few points, I feel like these are probably things that will either get easier as you adjust to the Gnome workflow, or can be remedied by extensions. I recommend installing the unofficial Extension Manager app my Matt Jakemen, which comes with a built-in search. Makes finding extensions very convenient.
The issues with the notifications sounds a bit weird. They shouldn't be taking focus away from the app you're working on. I use Thunderbird all the time and it never does that to me. Also, xfce-notifyd
? What is that running on Gnome?
Ah yeah, this will be why. Installing different DEs as the same user is pretty much guaranteed to create conflicts and generally should be avoided. It's always best to create a new user account for each different DE you want to try out so that it doesn't screw anything up.
P. S. Gnome includes a Wayland session by default, so you don't have to install it separately like you do with KDE.