Where's your 4^32 coming from? There are only 2^32 ipv4 addresses. You may be right that the first wave would just target port 22 and the second 2222 and so on, an actual attack would probably have some interesting implementation details besides that too for pruning or host retry or something else.

Why do you think at minimum you would have weeks? Run the numbers for botnets of various sizes with a measly 10 Mbps network connection each, it doesn't look very good. Under normal circumstances yeah port 26432 is no more likely to be hit than any other high port, but an ssh 0day bypassing authentication is an incredibly valuable exception where now trying everything can be worth it for a little while.