I use it with calibre. Works perfect. Even displays the book covers (no matter where you got them) as lock screen background of the kindle. Can absolutely recommend this!
Can confirm it works now (not sure when change happened; pretty sure I’ve been using since early 2022). I regularly use epub on kindle.
They did recently drop mobi (or at least threaten to - they send me an email saying they are going to drop mobi capability after I send one to my kindle).
Also idk about the newer ones but the old ones last forever. You might need to change their battery but that's not too hard. I got a kindle keyboard that's been going strong for over a decade now.
Also kindles work fine with calibre, you just need a different file format. Mine can read PDFs! (I do not reccomend reading PDF scans on a kindle)