I love Bitwarden and you can self host Vaultwarden. I’m not sure how OSS it is however.

I have Kuma setup on one of my VPS’ and one endpoint I monitor is on healthchecks.io.

This way Kuma can monitor whatever I need and if HC doesn’t get a “ping” every 5 minutes I get an alert from them. They have a free teir for 20 checks I believe it is.