That's really up to the software again. If you're not technically inclined enough to run through the code, that's fine, but you have to trust that other people are.

Go and search GitHub issues or this project by name for what you're concerned about.

Authentication is also not security, btw. It's just access. If you can be more specific about your concerns in your post, you may get more direct answers.

None. Dashy's authentication was famously literally security theatre even with Keycloak. You could just pause the load in browser and have full access to the config. Because it let you iframe whatever you could now do so with local services to enum. Somehow Jellyfin is unbustable though. So it's a bit of a crapshoot. Look at past vulnerabilities. Stuff like XSS unless stored you don't need to worry about, clickjacking, tab nabbing etc. On the other hand anything that's arbitrary file read, SQLI, RCE, LFI, RFI, SSRF etc. I would look at seriously. E.g. don't make your 13ft public because it can be used to literally enumerate your entire private network.

I'm not sure how much you can trust them, but you might want to have a look at Authelia

I'd like to do the same, but atm I use nginx to serve all the web interfaces... And keycloak support is either a plus subscription feature or made to work with hacky Lua scripts.

So for now it's security through obscurity, I got a wildcard cert and the pages are accessed based on subdomain. So afaik nobody has a clue unless they start iterating common subdomain names. (At some point™️ I'm adding proper auth though)