The dps requirement is actually pretty low if you follow mechanics and keep the demos in check. I did a +13 siege the first week of m+ when we were like 345 ilvl. Split in whatever makes sense for your composition and dps.

I also found that the speed was kinda random on the shit you had to dodge. Probably related to distance from boss but certainly came very close to catching me off guard a few times. Just move ASAP.

The demo spawn rate is fixed, possibly a more generous timer on first demo spawn to account for travel time across the planks?

Knockbacks would be from failed dispels. Spread from anyone who has debuff. Make use of personal dispels (priests, dark irons, etc) and have the healer prioritize as they see fit.