I gifted my sister my kindle this morning (it was unused for years) after loaded it with azw3 files converted from epub using calibre. The other options is to send epub via email. But I told her to never put it out of airplane mode and get it on WiFi because amazon has deleted my "documents" (that's what they called sideloaded books) before.

I own two kobos, a libra for fiction and an ellipsa for tech books and textbooks. Both have koreader on.

using https://www.amazon.com/sendtokindle it's trivial. I just bought the Greg Bear humble bundle (27 epub books), dragged all of them in one step, clicked the submit button and that was it. Books showed up on my paperwhite kindle in a moment. Couldn't have been easier. Books sent that way don't disappear from your device as far as I can tell - I've done similar large batch uploads for ages, and they're all still in my library, and available on all of my devices.