Not the parent and I will continue to use F-Droid but Obtanium is a popular alternative. It allows you to install apks directly from various supported sources (forges, F-Droid repos etc) so you typically use the apk that the app maintainer has produced in their CI pipeline rather than F-Droid's reproducible builds.
F-Droid would likely get APKs from the same place (if reproducible builds are on for the app in question). If this attack is implemented successfully, then that place was compromised as well, and Obtainium can’t do much here to detect that I’m afraid.
Edit: on second thought, they could pin certificate hashes like F-Droid does on the build server, but verify them client-side instead. If implemented correctly this could indeed work. However, I think F-Droid with reproducible builds is still a safer bet, as attacker would have to get write access to source repo as well and hide their malicious code so that F-Droid can build and verify it.
