While not an "app store" per se, Obtainium is pretty useful. It lets you manually import apps that you'd like to receive updates for, and can pull from different sources. For app discovery, I use Droid-ify.
I have found Obtainium to be quite unreliable and am surprised by how many people recommend it.
In my case it frequently fails to detect which version I have installed, fails to detect there is a new version, and even tells me an app is outdated when the version I have installed is higher than what it believes is the highest version. I currently use it only for 2 apps: Uazo's Chromium build (which is not on F-Droid) and Obtainium itself.
In case an app is on F-Droid I personally see no reason to get/update it through Obtainium.
Strange. I haven't encountered any major quirks with it in my use. I've had app gave me trouble but it was because I had installed a pre-release version of the app by mistake, but once I installed the correct version, it was fine.
I will say one criticism of Obtanium is that it's easy to get rate limited if you use a VPN, which is really annoying.
Yeah, that is just my experience and, of course, will vary with others depending on which apps you track with Obtainium, and maybe on which ROM you run.
I still find a bit odd how some people want to avoid F-Droid, being a reputable source that checks for anti-features and builds from source to make sure the final apk matches with the published code.
However, I am happy that there are several options to install and update FOSS apps in Android and derived ROMs: we can all choose what we prefer and have other options as backup.