For monitoring servers: Zabbix would work.

For APM across some popular stacks: Apache Skywalking might be worth a look (though probably with PostgreSQL instead of ElasticSearch).

For analytics: Matomo is good.

For uptime monitoring: Uptime Kuma is really good.

Consider a separate cheap VPS for the monitoring stack, whatever it might end up being, like something from Hetzner.

Poor man's dashboard: Get your desired metrics with grep/wc/whatever shell commands from your logs, and visualize with PyQtGraph/PySide (or even gnuplot, for an even lighter experience).

Check out Coroot, it is rather light, simple to deploy and understand

netdata https://github.com/netdata/netdata

Grafana doesn't take up gigabytes unless you're doing something crazy; it's about 100mb in my small business deployments. Which still might be too much for your $6 VPS! I would still highly recommend Prometheus, as it's native dashboard has gotten a lot better with 3.0

I would love to know the answer? I wonder if Prometheus and Grafana would work at this light level of compute?