Good luck getting grandma to install and use Graphene or Lineage on their 3 year old underpowered Samsung device that already works the way they know.
I think you’re making problems where there aren’t any. A device PIN or fingerprint along with Signal is probably way more secure than any family chat needs to be.
I’ve worked back and front end, and with a lot of developers, and I don’t think anyone would say they’re the same. Software devs are some of the most pedantic people out there.