Sorry, I had made a mistake. Wanting to create a major key starting with D# would end up looking like D# E# F## G# A# B# C##. The ugliness is even more stark.

- two double sharp keys - F## to refer to G, C## to refer to D, B# to refer to C

Yikes.

D# would be extremely rare. I’ve only ever seen G#, and that was a temporary modulation within something larger.

Most people write “x” for double-sharp, instead of ##, in order to match how it looks on a score.