Just stick to signal, matrix can be used the same way but it’s a lot more complicated and requires involvement in keeping your backup key safe, otherwise you’ll lose every encrypted message! Signal is similar to whatsapp. (which is obviously…a popular app…which should already explain the simplicity of setting it up.)
Our VMs are properly synced, it’s a known kbin bug but i haven’t seen if they solved it already. Once they do fix it i’ll update it.