One day, I moved all services I really wanted from a couple of random VPS to a nice little proxmox machine at home (and then added some more services, of course). That was the day I swore to document stuff better, and I'm pretty satisfied with how well I was able to keep up with that.

In the proxmox web interface, you can leave notes per container. I note down which service the container is running including a link to the service's web interface if applicable, plus the source, and a note about how it auto-updates (green check mark emoji) or if it requires manual updates (handiman emoji).

Further I made a concious effort to document everything into a gollum wiki running on that proxmox host (exposes a wiki like web interface, stores all entries as plaintext .md files into a local git repo - very "portable"). Most importantly, it also includes a page of easy to understand emergency measures in case I die or become unresponsive, which I regularly print out and put into a folder with other important documents. The page contains a QR code linking to itself on the wiki too in case the printed version might be outdated here or there.

The organization of the wiki itself (what goes into which folder) is a bit of a work in progress, but as it offers full text search, that's not too much of a problem imo.

Also using my password manager, keepass2 in my case (synced over webdav). A password manager should provide plenty of options to structurize. Password database is a part of scheduled backups, and always present on multiple synced devices, so a total loss is hardly imaginable.

As SSH keys were also touched as a topic in the OP, I just wanted to add I just found that there seems to be an addon for keepass that makes handling those even easier: https://lechnology.com/software/keeagent/ (haven't tried that yet).