A combination of periodic speed tests to multiple reliable endpoints and local netflow statistics are needed to understand network performance. It's relatively easy to set up things like librenms, cacti, and iperf hosted on a vps with dedicated throughput. There's a slew of open source and easily accessible software available.
Public speed tests are inaccurate at best, and very often downright misleading.
They're good enough for a quick test to see if you're online and a very rough indication of throughput. Even repeated tests to higher quality services though, like fast.com, are not a good benchmark.
Set up your own network monitoring stack - once you've built that, you have an anchor point for your home networks that you can riff on instead of being dependent on your isp's default setup.
In this case, you could track real-time performance of starlink and be able to trust the data without wondering if there's congestion between fast.com or speedtest.com servers and the local starlink ground station. If you can talk to the datacenter owners in your local fiber hotel where the starlink ground station connects to. They may have an iperf server, or allow you to host one there, giving you another test point when diagnosing an issue.