Someone made a compilation of academic reviews and blogposts here: https://community.signalusers.org/t/wiki-overview-of-third-party-security-audits/13243 but none of them seem to be real security audit reports, ex. compare with real security audits to Delta Chat: https://delta.chat/en/help#security-audits
You can always look at their history “complying” to government orders to hand over user data.
https://signal.org/bigbrother/
No company is going to break the law for you, so live tests seem about as good as a security audit.
IIRC by US law they are not allowed to disclose requests from US gov itself
I would rather prefer real security audits