It is very snappy (unlike how I remember desktop Thunderbird all those years ago), and seems to work well so far, though it's a little bare-bones feature-wise. One thing that is unfortunate is it doesn't appear to be able to recognize Fastmail's "pinned" messages and map them to "starred" messages. That alone will keep Fastmail's own app on my phone.
Fastmail also has different actions for marking as spam and reporting phishing; I'm not sure how effective the latter is, but of course Thunderbird doesn't support it, as presumably that's a Fastmail-proprietary thing.
I'll probably keep it and see if it can serve as my daily driver, with the Fastmail app as a fallback for pinned messages for now.