Most third party app stores suck on stock Android. Even GrapheneOS doesn’t recommend using them. Don’t get me wrong, I love F-Droid, Aurora Store, and Obtanium, but without integrating them as system apps or Shizuku, updating apps sucks ass. It is also much harder to detect if an app author were to become malicious with things like Obtanium, and even Aurora has the potential to have it’s anonymous accounts become compromised and create “fake” apps that appear legitimate from the developer console. The most promising option is always going to be something completely independent from Android, like PureOS.