It is indeed trivial to "side-load" ebooks onto an older Kindle; you plug it in via USB and drop the files into, IIRC, the "/documents" directory.
Kindle, for many years and I think even now, does not actually support EPUB, for likely unethical reasons you can probably guess; I believe the Kindle email address method hides a conversion to the AZW / AZW3 format.
Kindle, for many years and I think even now, does not actually support EPUB, for likely unethical reasons you can probably guess; I believe the Kindle email address method hides a conversion to the AZW / AZW3 format.