I understand your point, but to add to the discussion:
With android, you can use the F-Droid app store, which have "source builds". Source builds are apps built (and signed) by F-Droid, so you only need to trust the F-Droid team instead of each OSS app developer (and you can use alternative repositories if you don't).
So OSS can be the solution.
Sadly such solution is not available on the locked-down iOS ecosystem...
With android, you can use the F-Droid app store, which have "source builds". Source builds are apps built (and signed) by F-Droid, so you only need to trust the F-Droid team instead of each OSS app developer (and you can use alternative repositories if you don't).
So OSS can be the solution.
Sadly such solution is not available on the locked-down iOS ecosystem...