IONOS is vastly underrated.

Influx/telegraf/grafana stack. I have all 3 on one server and then I put just telegraf on the others to send data into influx. Works great for monitoring things like usage. You can also bring in sysstat.

I have some custom apps as well where each time they run I record the execution time and peak memory in a database. This lets me go back over time and see where something improved or got worse. I can get a time stamp and go look at gitea commits to see what I was messing with.