Look into Ubuntu Touch on one of the newer supported phones. It uses Halium though, so I'm not sure it fits your definition of True Linux.
It checks all of your bullet points, but make sure the model you are considering actually has VoLTE enabled for the port.
The Waydroid container runs Lineage, a degoogled android based os. Many apps require Google play services to run, or they do other checks that fail in that environement. Most stuff from F-droid will run.
Signal does not have a native UT app at the moment, but some use Matrix bridges to send and receive messages. Others run it in Waydroid, or do experiments with the cli version. The first works very well, but you need to find/make a bridge host that you trust.
Oh, it feels very nice to use! Most of my troubles these days stem from me experimenting and running the devel version of the os. I can go days between serious issues, and the issues that do appear are never deal breakers as they don't tend to affect basic phone functionality. It feels great and it is way too much fun.