@Finn666 I'm not the resident Rolex expert around here. Someone like
@louisbosco might be able to advise better.

The GMT looks fine on you. It's a 40 mm case, and the Sea Dweller 4000 is 43 mm, so I think that'll work too. Provided the lugs don't project past the sides of your wrist, you'll be fine, and you'll adjust to thinking that the size of the watch is fine after a few weeks.
A bigger issue will be thickness. Some diver's watches can get quite chunky, an this makes them harder to slip under the cuff of a shirt.
The problem with getting a new Rolex is going to be supply. A huge amount of stock is
being bought up by flippers, who then sell them onto the secondary market. The only time I've seen one of new GMTs, a Daytona, or a Deep Sea Sea Dweller in Melbourne has been in a dealer's window.
Lastly, there are rumours that the black and blue "Batman" GMT is going to be discontinued. So if you want that particular colour, you might want to move fast.