EPUB reader
Can anyone recommend a good one?
I'd like one that can kinda "save" my progress not only locally. Maybe an app account or something that integrates with a cloud service?
I had an ancient one very early in my smartphone days, but as I lost/had my phone stolen I would lose all my progress and miss the pages of the books I was reading.
On f-droid, KOReader is the absolute best. It's got every feature you can imagine which can be overwhelming but empowering. It has a couple of littlebugss here and there but if you're wanting FOSS and a feature rich reader you can't beat it.
Cloud account? Probably not an option with most FOSS tools. I suppose you can syncthing the koreader directory which is where save state and other stuff is located.
I bought Moon Reader Pro a million years ago cheep on sale, and it has been great. Still love it, but I like to check out open source readers every once in a while.
Lately I have been using Librera FD. This open source version on f-droid unfortunately has the network features (sync/opds) disabled. The play store version has those enabled. The interface is a bit unusual, but it's a powerful and flexible reader.
I use KOReader and it has a built in page syncing. You register within the app and save the positions; then you login on another device, copy the same file (for example I use the built in cloud connection to access Nextcloid/Dropbox and download the book) open the same file and it is synced.
Librera has a Dropbox integration (possibly in the pro version) and it may sync. But I never used such features since I prefer KOReader on ereader.
Not FOSS, but Lithium on the play store has what you need and the app is free of tracking and ads.
I find open-source apps in this department to be still lacking.
It is not possible to make free open source app without tracking to save in the cloud.
"Book Reader" from f-droid has "sync folder" option, but I never tested it. I guess of you sync that folder with some other app (like syncthing, or google drive) you can have it in other places.
A simple one i like is booky mcbookface but it doesnt really meet your requirements. I have tried librareaader but find it complicated to use as a casual user
Koreader or librera
I use Moon Reader Pro vom Play Store . You can use Clouds like Google Drive or Dropbox.
Aldiko saves your progress in epubs you are reading. I use Calibre to organize various collections.
Haven't seen one that uses a cloud service.