As someone who hosts their own email, I dislike Gmail as much as anyone. But your issue is this:
> I go to check my email in Gmail’s web UI.
You quoted the very first sentence. They acknowledged your point later:
> Sure, I could switch to a different mail client and never see any of these language model features, but my experience these past months has left such a bad taste that all I’m looking for now is a clean break.
The brand/trust is ruined for OP even if there are workarounds to not directly see what Google's doing anymore.
Gmail has three main features that matter (to me at least) -- and they are huge, very important features. And as much as I don't like this fact, using their official web or mobile clients are the only way to get them:
1. Accurate, deterministic, fast search of your email
2. Whatever they call the categorized inbox, I use "Primary," "Promotions," and "Updates."
3. Labels implemented as labels, not mapped clumsily onto the "Folder" concept.
If I were told I had to not use the Gmail UI, I would 100% switch to another email provider immediately, as using Gmail the service with a vanilla IMAP client is way worse than just using a normal email host with the same.