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.

Right now, I’m using Obsidian. I think I’d like to transition to keeping docs in a wiki, but I worry that it’s part of the self-hosted infrastructure. In other words, if the wiki’s down, I no longer have the docs that I need to repair the wiki.

Synology has the best systems of their kind. I’d go with them for pre-made solutions. Their UI is simple enough for most folks to understand.

Backups. Backups. Backups. Focus on what you can reliably do. If you can’t make a service bulletproof, then maybe it’s not ready for everyday use.

Keep good notes. Notes tell both what you did and why you did it. Keep track of what problem you’re solving or what goal you’re working toward. All of this will help when you do look for a new IT provider. Use your notes to help the business define requirements for them.