Johnny's got the tools part covered, so here's the rest.
The "repository of nostr knowledge" question has a canonical answer: the NIPs, in the nostr-protocol/nips repo on GitHub. And for the per-relay features question there's something better than any directory: every decent relay publishes a NIP-11 info document over plain HTTP. Run curl -H "Accept: application/nostr+json" against the relay's URL and you get the exact list of NIPs it supports, plus its limits (auth required, paid, max message size). It's a live feature list and it can't rot the way a directory site does.
For the config pain: stop trusting the client as the source of truth. Keep your own relay list in a plain text file (or a tiny git repo if you want history) and treat every client config as disposable. When something breaks, the fix is a copy-paste, not an archaeology dig.
For the client question itself: Damus iOS has the cleanest relay management view I've seen, the NIP-11 spec even references its relay list UI.
