Aurora Store is way too unreliable and is nearly unusable. I have been using APKMirror so far and while it is okay, it is somewhat annoying that Obtainium can not directly download from there whereas it can with APKPure. I wanted to know the security level of APKPure in comparison to APKMirror.
If you can’t get a packaged apk directly from the developer/publisher, or from a trusted repository like the play store or fdroid, I wouldn’t resort to third party sources like these. If you can’t compare the signing signature of an apk from an untrusted source to that from a trusted source, you can’t be certain that what you’re installing hasn’t been tampered with.
F-Droid is not what many think it is. Check this out for some interesting reading.