Just looking at the picture, that grab bar is not mounted like a stanchion. It is bolted from the bottom. You can get to the front supports from underneath the helm but not the four side ones.
The grab bar looks like it was put on before the flybridge was installed. To get the last four off you will have to remove the flybridge which you undoubtably don't want to do.
You could just hacksaw/sawsall off the last two supports on each side, then later reinstall them using one of these stanchion support bases- https://search.defender.com/?express...ion%20base&s=1
It would probably be just as strong as the original even if you just used lag screws into the fiberglass as those bases have four screws to hold them down.
But I am not sure that removing the grab rail will get where you want to be. Talk to a boat hauler about your route, what the height of the keel above the road will be and the maximum height you can live with. Then carefully measure and see.