If I have to dig through third party clients in order to trust a system, then it's clearly a shit system. Signal > anything else, especially telegram, which can burn in hell for all I care.

I don't see the difference with Signal here. In both cases, the only reason why you know that they do E2EE properly is because you (or somebody else that you trust) has audited the client code and confirmed that it does indeed do E2EE.

Nor does it require a third party client. In fact, in this regard, Telegram official client is slightly better because they have reproducible builds for iOS, while Signal, last I checked, does not (they do have them for Android).