I'm using DO's Spaces (S3) as file storage. I also suspect very strongly that it is the main source of the slowness of the nextcloud (but it is super cheap!).
The main pain in the ass is having to update every year or so. It usually works flawlessly, until it doesn't and I have to run a few manual commands. Other than that, it just works. 100% recommend it.
Mine is a personal instance, being used by 5 people (family). I've had the nextcloud and its postgres database running in a 5$ instance for years, no problem (I did add a bit of swap for the DB).
True. The Nextcloud docs suggest at least 2GB of RAM.
On the other hand, mine is running at 400MB right now.
I guess it also depends on the frequency and how it is used. We mostly use the apps on the phone and file app integrations (Ubuntu lets you access the files from the Files app).
It is true that it is somewhat slow when loading the website.