everything we do happens because of something other than us, and all things in the world are moved by things other than themselves, and that everything that happens is caused by the past, but that the past itself happened because of nothing.

NOT "because of nothing" but rather "because of unknowable".

If you want to suppose some sort of "prime-mover" or whatever, I don't see how that makes any difference to anybody.

Deciding to call the "unknowable" a "prime-mover" is simply a label-swap.

It changes nothing and means nothing.