Except Kindle isn't supporting ePub. You can't sideload an ePub on to a Kindle. All you can do is e-mail an ePub to Amazon, and they'll convert it to AZW3 and load that file on to your Kindle.
Further the conversion that they do is notoriously bad and often results in unreadable books. So you often have to tweak the ePub files before sending them to get a decent result. Calibre is essential to that process.
Notably they don't display book covers for converted epubs in your library -- clearly hamstringing the conversion so folks aren't too tempted to rely on books from external sources.
The AZW3 proprietary format is not something I want to struggle with in my day-to-day -- if anything goes wrong, it's a rabbit hole of reverse engineering. I like copying media to my ereader using cables, because sometimes I don't have a solid internet connection or it just feels wasteful to send a bunch of big files over email (if I'm on a tethered or metered internet connection, just sending a couple dozen books is easily in the hundreds of megabytes up and down).
Kobos are a much friendlier ecosystem, and are what I recommend to all family members who don't want their device shackled by Amazon. Easy to hook up to local libraries to borrow books on the fly, easy to copy DRM-free ebooks from online stores.
I have a Boox tablet myself because I love notetaking in nonfiction books and hand writing my notes in work meetings and journals.
Further the conversion that they do is notoriously bad and often results in unreadable books. So you often have to tweak the ePub files before sending them to get a decent result. Calibre is essential to that process.