Matrix also does have a pretty big problem with meta data. By default it stores a ton of meta data (at least the reference server implementation does) and I am not sure if this is even a solvable problem without redesigning the protocol. When opting for an alternative to Signal, XMPP is probably the better choice.
No, even the earliest Ryzens support ECC reporting just fine, given the motherboard used supports it, which many boards do. Only the non-Pro APUs do not support ECC.