I also switched to Fastmail 6 months ago, and I'm content. It's not bad, but the UX is nowhere near Gmail.
I didn't understand how useful was automatic email classification (Updates/Promotions/etc.) until I stopped using it.
For updates, I've got a filter based on substrings in the subject like "receipt", "invoice", "latest bill" which does fairly well at catching that transactional type of email.
It definitely takes more time than Google's implementation, but in curating the filters myself I've found I've been left with a cleaner, more organised inbox.
[0] https://protonmail.com/support/knowledge-base/sieve-advanced...