ePub is the standard format. I’ve made sure to convert everything I’ve bought back to ePub without DRM.
I read a lot in Japanese. One nice benefit of this approach is that all the dictionaries and other language learning tooling is just ready to be used.
"Locked to Amazon unless you jailbreak" is overselling it imo. You've always been able to (very easily) sideload DRM-free ebooks and read them on your kindle.
Since "reading ebooks" is ostensibly why you'd buy a Kindle in the first place, I'm not sure what more you need.
"Easily" does not apply to grandpa and huge swathes of the human race.
"Type 'kindle' in the search."
"Alright, hold on... do I want kindle.com? I'm on Amazon, now where do I go?"
A slightly more experienced fastball realizes grandpa has opened his never-updated Internet Explorer on his old Windows box.
Maybe your family is tech-savvy, but there's many who aren't.