I know it's not self hosted but why not Signal? Matrix is demanding on a SBC and your family would probably get the 'unable to decrypt message, please re-verify keys' error that happens in encrypted matrix group chats and Element does not have the best UI especially if you want your grandma to use it.
What's your source on the reverify thing? I use matrix a lot, and this hasn't been an issue I ever experienced anymore since they introduced cross-signing a couple years ago.
Same goes for the common clients such as element. It has been clunky in the past, but after the past major overhauls ( also years ago now) everything has been silky smooth for me, if not better than others. The one thing left I prefer from Signal is the one-time photo share.
Matrix is great, clients are great too, only the server part still is annoyingly complicated and messy. Would only recommend that for tinkerers, on that case it's a great path to learning about the complexity of addressing lots of security concerns that others gloss over.
Edit: to add - there's a reason why the French government and the German military decided to build their secure internal IM infrastructure on Matrix. Obviously they are hosting their own private network, but if the concept is good enough for European government and military, it is an indicator for quality especially in terms of security and privacy.