Fluffychat (matrix), Deltachat seem really good but good luck getting anyone to change. Let us know if you figure out a way to persuade people they should make any effort or try something new.

I would say Snikket if you want decentralised, Signal if you don't.

Most of the options listed are easy to use. The difficult part is getting enough users to adopt the new system initially to motivate the rest to migrate as well.

You're asking for a literal unicorn. I haven't found one after years of searching.