In case anyone else who's in my shoes reads this, I'll unpack JSWolf's response as best I can. Calibre doesn't output KF8 format per se, but there is a KFX-output plugin that seems equivalent. Here's the info:
https://www.mobileread.com/forums/sh...d.php?t=272407
I converted to KFX and sideloaded no problem, but I wonder why I went to all that trouble (installing a plugin and a preview installer; whew!) when AZW3 seems perfectly compatible.
I also tried Send to Kindle, and emailing to my @kindle.com address; the latter both with the original .KFX extension and renamed to .KF8. None of those worked.