For what it’s worth, the multi-user experience in my case has been pretty seamless. Here’s my setup if it helps anyone:

My roommate and I both have separate steam accounts (it sounds like you may be looking for a ‘child’ account or something like that, those may be a thing but I’m unfortunately completely unfamiliar with that, so ymmv if you use that).
We set up family sharing between us to access each other’s games, but did that I think entirely on a computer via that steam client. No pins or anything were necessary iirc, just a slightly convoluted sequence of logging in and out of steam on the same computer and clicking the needed ‘family sharing’ buttons.

Then I set up the deck with my account, logged out, and had my roommate log in. There’s an option somewhere to start the steam deck at the account select screen every time it turns on rather than automatically logging in to the last used account.

It sounds like most of the difficulty is coming from the family sharing setup. Like I said, I’m not knowledgeable on if steam has ‘child’ accounts that can be linked to other accounts, if so it’s possible that none of what my process was like applies.

Hopefully that’s at least somewhat helpful