It helps to keep a backup account on another instance, in case there is an issue with your main server. You can export your settings and subscriptions from your main account and import to another Lemmy instance.

Each instance of Lemmy has their own list, like so: https://feddit.org/instances https://feddit.nl/instances

Without an installed desktop environment on your Pi, you will not be able to use GUI apps, not even remotely. You would have to install it on the Pi first.