Everything on the internet is being corrupted with adding cryptocurrency scams where they absolutely don't belong, it turns Signal from an obvious recommendation into something that makes me hesitate. There's something to be said for focusing on doing one thing well, and that doesn't mean turning a communication platform into a kitchen sink.
Anyway Signal is itself pernicious by being tied to a phone number and to Google Play services, and by being very choosy about who gets ports.
I had high hopes for Matrix, once they got E2EE, but they have flubbed that by requiring a very heavyweight bounce server that won't fit on (e.g.) your typical home router or super-cheap cloud VM. Matrix should enable a place to keep your message archive independent of the bounce server, and allow gatewaying a non-public storage service via the lightweight bounce service.
But Element.io's business model is tied to heavy-weight bounce service.
And as others have said, Synapse really isn't that heavyweight these days (thanks in part to the performance improvements driven by Element!)
It's not tied to Google Play Services, you can download a standalone version from signal.org. As for phone numbers, the developers have been working on getting rid of them for a while now – there's already a good amount of code on GitHub.