So, get better parents?
All you need to do is convince them that it's for their good, if they can agree on that then the technicality isn't difficult.
I suspect one of the most difficult to address is getting it to successfully evade the random vendor-built aggressive battery saving settings in Android. WhatsApp is usually whitelisted from those so "just works". If you don't do similar things for Signal, it will stop picking messages up/notifying you after a day or two of inactivity.
Unfortunately, that leaves me with ... nothing to recommend people switch to.
Whatsapp allows media from specific chats to be auto saved to photos. It is a very important feature, not sure how these aren’t making the priority bar.
My parents (and my extended family + friends) have been using signal since the Snowden leaks.
All I had to do was ask :) It's not really a hard app to use either, I had a few calls from them when they introduced the PIN stuff and when I or someone else have switched phone but it's not comprehensible. My parents are closing in on 80 now.
It sounds like your parents are very unusually observant! I have had coworkers much younger than that who cannot be bothered to read messages like that.
I'm not kidding - it worked amazingly well for us.