People on the dom constantly twiddling their bids/offers size by +/- 5 contracts - why?? and what platforms will allow you to do that as effortlessly/purposelessly as they do?

Probably a lot of what you see is large traders and they may be using bots to place/cancel/move trades and they usually have many accounts. Some brokers even allow them to trade both sides of a market at the same time.

Some of the trades may be just spoofed and not actually an offer to buy or sell just something to make others take a trade or close positions.

I can't move my trade as quickly as them but I don't need to.
But that being said, I can move or change my offer or bid in a very short time. (less than a second)

Some of the positions may be on a co-located server and there is almost no lag from the time the order is place/moved until it hits the exchange.

I would not worry about how fast the orders are moved.
It is kind of like a fisherman with a fly rod. If there is no strikes on the lure, the fisherman will twitch the lure or move it quickly to get some action.

My advice is 'don't be the fish'. Wait for your entry and don't be distracted by the moving of the orders.

just purely curious why'd they do it non stop as if just for fun like 203 -> 202 -> 204 -> 203 on and on