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.
I've made the same switch and had the same thought. In response I just unsubscribed aggressively from most everything. Much cleaner mailbox now, and I really never took any action on almost any promotional/update style email anyway, so less noise overall than Gmail now. It's actually been a good switch.
Not sure if Fastmail's filtering is as extensive as ProtonMail, but it seems there are several common headers that do a fairly good job of sorting emails into types. ProtonMail suggests filtering based on these headers[0].
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.
I know it's not the same, but after going on an unsubscription drive, I've had pretty good success with simple address-based rules to move new mail to folders.
