Obtainium is one that I have been using for over a week now. It keeps all my FOSS apps up to date. I have 30 apps added now, and 80% of them are now updated directly from their github page!
The easiest way is to look at your Installed apps through F-Droid as it will show both apps installed through F-Droid and apps from the Play Store that are available through F-Droid. You can copy-paste each source link into Obtainium.
I use Obtanium to keep Liftoff and Thunder up to date, since the updates come faster than they do from the Play Store :) Very easy to set up...even if you originally installed from the Play Store, you can still use Obtanium to update going forward.