Splitting the developer and the packager doesn't inherently reduce the chance of a malicious update any more than using a VPN reduces the chance of being snooped on by an ISP. All it accomplishes is change who you have to trust to not be malicious. You might have good reason to believe that you can trust one party better than you can trust another, but unless you're building the package yourself there's still no guarantee that the package that you install is built from the source code you can inspect.

It's all based in trust in the packager and only the packager—there are no checks and balances. The only reason why splitting up the responsibilities might help is if you find the F-Droid maintainers to be inherently more trustworthy than the Signal developers, not due to simply separating the concerns.

Without reproducible builds, this just means you have to trust the packager instead of the developer. Sometimes that's a good trade-off, but you still haven't really solved the problem, just moved it.

With reproducible builds, you don't have to trust the packager or the developer as long as you trust at least one person who reviewed the source code.

Packagers have proven to be more reliable. Sometimes they make mistakes but there's no case of a packager ever selling out (correct me if I'm wrong.) On the other hand, there are numerous cases of developers selling out.

That does not solve the problem. A country can forbid F-Droid and Debian and anything else that is not in a short list of vetted app stores that comply with the law of that country to backdoor everything.

A sovereign country creating and enforcing domestic laws is not a problem that can be overcome with software.