I like all these ideas!
I was thinking of maybe taking the "Ablative" Limitation that you can apply to defenses, and applying that to the DCs of their attack, to represent the waning firepower as members get injured or killed. That might have the effect I'm looking for, but it might take some fiddling around. Allowing them to break past the speed limit for the campaign is also a great idea! Definitely a good way to simulate having more bodies than just a single Space Marine or what have you.
This list is full of great ideas, thank you for your contribution! 😃