Do you use the 1 1/2" drill hole universally at each hole at the base, assuming of course that there is rot to be fixed?
Usually I use a 1/4 over sized stiffener epoxied to the deck , and then a single 1 1/2 plug works fine.
Where an owner does not like the "look" of the added plate , I prefer a 3/4 hole for each thru bolt and a center plug of a size that looks OK. All holes are epoxy filled and redrilled.
Since we will not attempt a gel coat match , this is as good as can be done.
Sometimes the GRP pad can be hidden with a teak pad and is OK with the owner.
For a deck mounted davit , we will epoxy the GRP plate in then lay up a GRP laminate on top , as the loads of a dink full of rain is high..