It may be beneficial to have a separate sub-board for generic guidance and issue reporting on installing apps.
In addition to APKPure and the Tauon Home webpage, I was able to install Amazon AppStore and use it to install apps.
I installed Google Play Store, but it requires a sign in and clicking the button to Sign In does nothing. I suspect it is expecting the device to be associated with a Google account, an Android phone is.
Although access to the Google Play Store is not completely blocked. I downloaded C64games, which has an option to install Fredo C64 from the Play Store and that works. So it should be possible to get Play Store to work with a little hacking.
APTPure wanted to update a bunch of the included apps but the installs failed. I had to delete the apps from the launcher and then it could install the new versions. Is this expected behavior? A thing to be aware of, if so.
Almost all open-source software provided in the firmware is taken from F-Droid site and is signed with their keys. You can update the software through G-Droid app, as it uses F-Droid repository. I don't recommend you though to update Brick Games and KeiKai Launcher apps as they are custom builds for Tauon OS.