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.
3 is a result of Gmail's imap implementation, and can be squarely blamed on Google. They did this intentionally. Instead of mapping the your tags as imap labels, they mapped them as a folder structure.
While it certainly shouldn't be necessary, I do feel like it should be possible to work around this in a mail client, however; don't emails these days all have unique ids? If not, storing a hash of the message content+headers/timestamps for every message should be enough to keep a local index of what each message is tagged as (given the implementation where each message shows up in multiple places). I can somewhat understand Thunderbird not wanting to make compatibility shims for specific vendors poor implementations.
I wish (more) standard clients aggressively supported these to give an experience as good as the Gmail UI in that way.
But there are a lot of IMAP protocol features that most clients (especially consumer ones) don't even bother with. Server-side search being the #1 worst offender.
I did used to keep my whole email archive on my PC and back then it wasn't too bad to use Apple Mail to search (or Outlook for Windows way back before that!), but my fullest email history is basically only in Gmail now. This is my fault, but as a result, search essentially does not work anywhere for me but in Gmail's Web UI, unless it happens to be in a very recent message that happens to still be cached locally.
Thunderbird works well for my needs. I just want to see my emails categorized in reverse chronological order. I don't expect or want any kind of filtering; I would just Sieve filters for that (running on my own server). Perhaps I am just old fashioned. Should I want AI assistance to write an email, I would fire up a local model such as GPT-OSS. Local models are more than capable for trivial tasks like this, and a smaller model on CPU only would also work.
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.
> I go to check my email in Gmail’s web UI.