The fstrim timer is activated by default.
Many EOS users use pamac, it can be installed from the AUR. However, I would warn you that since it targets Manjaro, there are times where it is broken on Arch or won’t build.
Yes. The Arch wiki has an article on creating a local mirror. That being said, unless you have a lot of machines you may actually find a full local mirror uses more bandwidth since you will be mirroring thousands of packages you don’t use.
There are several versions of the Manjaro breath theme in the AUR.
We currently use bash by default but EOS is all about you customizing the system to your needs so there should not be a problem with you switching your default shell to match your preference.
The instructions probably need to be updated for Cassini.
Calamares, our installer, doesn’t support zfs mirroring. The zfs support isn’t sophisticated enough for that. You might consider adding a feature request upstream at Calamares for that.