I have the exact opposite experience and find Fastnail's spam detection quite superior.
There is the occasional spam.email getting through - maybe 1 per day - despite the fact that I have quite a number of custom domains with catch-all email addresses.
At the same time I really never have to check my spam folder for false positive, everything in there is literal spam so I forget checking it for weeks on end.
Whereas on n Gmail I always need to check the spam folder, Google completely overdoes the filtering it and a lot of wanted emails end up in spam.
Works for me on a 2018 Moto w stock Android 10.
My color options are (translated) natural, boosted and saturated, w saturated active. Seems to be the factory default.
CyberChef - The "Cyber Swiss Army Knife" - single file web app for all kinds of conversions, encryption, encoding, compression and data analysis
https://gchq.github.io/CyberChef/