The qartz door is extremely useful as a portable mini-base and it's perfect if you play alone, however can be a bit "clunky" if you play with friends since everything you have in your personal pocket is basically inaccessible to them.

I would suggest adding a way to let selected people have free access to your personal pocket, or a way for 2 or more people to fully share just one personal pocket.

Additional context: I am trying to build a custom modpack fo my server that would strongly encourage exploration and a semi-nomad life style.
To help achieve that I'm planning to make the quarz dimensional door available very early, so that players can have a movable mini-base to bring with them while exploring.

This is one of the conversation topics that came up when we were talking with @StevenRS11 yesterday.
I had the idea that we could do this with some kind of permission manager block in the middle of a dungeon, but Steven came up with the idea of making this an extension of "lockable Rifts" that you would be able to open with a certain key.

Specifically, one would craft a key and use it on a Personal Rift, after which the key would be able to grant another player access to their Personal Pocket when entering that/a Personal Rift holding that key.

Is it not possible to create a second entrance to the pocket using a rift signature?

Yes, but then everyone can just walk in and out.

You could give the signature to another player and have them set up the other end in their personal pocket. But that's kind of clunky.