F-Droid

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.

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

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.

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.