Not trying to be critical, but I don't think you are putting enough resin on that FG and pressing out all the air. It should be all like those transparent parts in your picture, those white parts have a lot of air still in the glass.
To get strength, you will need to bevel grind back about 6" on both sides of the cut.
Overlapping the tabbing on each layer of 1708.
Then grind off the gel off the whole transom, and about a foot forward of the transom on the sides and bottom. Followed by another layer of 1708, then fair smooth and flat.
The only reason I wanted to use the old glass is I reglassed both side with 2 coats and it's pricey for the materials but if it's better and easier to not use it I wont