Sorry, you're absolutely right. The overhead of it was more than I cared to do (needing to use Calibre instead of a drag and drop of a file), especially since Amazon would then report my newly loaded books back to themselves. That's the part that I really didn't like.
Shame to hear about your Kobo's battery. FWIW, they have great repairability (in newer models at least). That said, the Kindle's battery does smash the Kobo's in my experience as well.
You can drag-drop the file from the file explorer, at least on my Kindle (2022). I think the OP mentioned Calibre because sometimes you need to convert the file for Kindle if you have a bespoke format.
Shame to hear about your Kobo's battery. FWIW, they have great repairability (in newer models at least). That said, the Kindle's battery does smash the Kobo's in my experience as well.