the only way to make it usable again is to switch to the plain html mode, that uses no javascript, loads very fast, and reminds me a lot of the first gmail ui, back from ~15 years go.
https://chrome.google.com/webstore/detail/simplify-gmail/pbm...
Injecting non-google code inside my gmail window sounds like a very dumb idea to me, honestly.
I already trust google very little and keep my gmail inbox because I'm basically being coerced/forced to (no gmail = no apps on android)... The last thing I want is adding another entity to the picture.
This helps avoid app-specific passwords when you use 2FA, and lets users use their regular sign-in flow (which could include enterprise SSO, TOTP, U2F key etc).
I imagine that there's the ability for Google to set some cookies as part of that process, although knowing Apple, would not be surprised to learn they had sandboxed that instance of the browser, to prevent cookies persisting into regular Safari.
IMHO, it's also the best if you're using Gmail: Google has been breaking its UIs for years (both on the app and on the web), especially for people like me who try to use proper quoting and signatures, or to send plain text emails. Apart from some glitches with reflow, K-9 does that very well even with Google as email provider.
I can't compare against Microsoft products since I've never used them on mobile, but seeing how horrible Outlook is on the desktop for plain text emails (and that's not an hyperbole), I bet K-9 is better as well.