I tried and gave up on getting Keycloak to use less memory. 500-1500 MB for a server with less than 10 concurrent users is ridiculous. And that's even using an external database.

What did you try? As a last attempt you can just check visualvm to see the actual memory use, add a few percent headroom and set that as heap size.

We were paying a million a month for a custom high performance GC for a little bit but we were able to get off that with a lot of development effort and get our five 9's latency under control.