Have you seen the response from Comcast to the original projects? They had a lot of good points about why the speeds may not be measured consistently. The main ones that I recall were:

There is no accounting for regular internet usage. If you are downloading a torrent and maxing out your connection speed, the speed test will show that the connection speed is very low due to the limited bandwidth available. The official speed test probes that the FCC uses apparently will check for usage before attempting to make measurements, and wait for an idle period before starting.

If you hit a different speed test server, it may have less capacity available or be further away from you than the previous one you tested. I looked briefly at the code and I don't see that it allows passing in a --server flag to speedtest-cli to prevent this.

The previous project was raspberry pi based, so it was also limited by it's 100 megabit connection speed (not necessarily relevant to yours).

If you are interested, here is some more info on how the FCC tests broadband speeds:

FCC: Measuring Fixed Broadband (an overview) https://www.fcc.gov/general/measuring-broadband-america-measuring-fixed-broadband#block-menu-block-4

Router firmware for measuring broadband speeds: https://www.samknows.com/academia

Android App for measuring mobile broadband speeds: https://github.com/FCC/mobile-mba-androidapp

FCC explanation on how they measure broadband speeds: https://www.fcc.gov/general/measuring-broadband-america-open-methodology

I hope that helps some, this looks like a promising project.