Look into Single Sign-On services (SSO) like Authelia, Authentik, or KeyCloak. Most SSO tools do the sorts of things you’re looking for. Some will talk to the native UNIX user store. I do agree with the others, though: if you’re this far along, then it’s time to spin up LDAP and SSO, but this might be the same tool in your case.
Fewer lists are probably better. I have several, but I only use the default reminders list and a grocery list called “grocery”
I have shared lists with my family for things like school and medical (grocery is shared too), but I’m the only one that looks at them, so I’ve quit using them.
If you have a lot of lists, then you’ll have to decide which list to add the reminder to, and that’s extra friction that you don’t want.