Let them stick with Molly/Signal, that will give them a lot of privacy, and nothing for them to figure out how to use.
Leave SimpleX for people more skilled to handle how to do configurations. SimpleX does have superior privacy over Signal, but mabe they can't do SimpleX. Take it in stages with what they can handle, don't jump to the end.
I'm not willing to Matrix and I don't recommend anyone use it if they wat privacy and anonymity. I'm content only using Molly and SimpleX with everybody I know and no other apps or messaging services.
I wish Mastodon had encrypted DM's. I've been focusing on using Mostodon as my main place for media.
I can't find a Matrix client in F-Droid to use because when I turn off all of the anti-features, it seems Matrix is not an option to install something.
Have you not tried SimpleX yet? If not, I would suggest when the new release for 5.2 is in F-Droid you should install it. I could give you an invite link to message there but I don't want others on here using the link.