[Quoth]

epub2 for Send to Kindle, but it ruins your privacy, wastes energy, Amazon gets a copy forever and it's slower.
"Cloud" is a marketing lie. There just servers owned by other people.

If you are using Google / Apple / Microsft email they take a copy too.

azw3 for USB, unless K1, K2, DX and DXG (which likely don't work well with Amazon now and only use mobi).

However Standard eBooks use overly complicated formatting, so you may have to simplify it. I convert all their epub2 to epub2 with Calibre filters.