Hello, this subject has been discussed so many many times on this forum. You should give a look at the threads.
I think I can summarise it that way :
- for open source apps, Obtainium is a great app, although not a real store,
- for apps you can't reach through Obtainium, the most secure store is Google Play,
- if you absolutely want to avoid Google Play, the only alternative is Aurora Store, although it just allows you to address the Google Play base,
- in a tomorrow, Accrescent might be an excellent tool, with great security and privacy features, but it is not production ready yet.