In order to make it strong enough for a rip-off the next time that you hit the sand with your rudder, I would suggest the following:

You will get the necessary strength only if you laminate the complete transom surface with an overlap on the inside side of the hull!

So there you have to taper the laminate to not disturb the streaming flow of the water too much.
Very important is to round-off (before) the angle between the steep end of the innerside and transom itself; so make a round edge. Use the complete thickness of the sandwhich hullplate. (Otherwise you get troubles with laminating)

After that laminating, you can add two or more layers laminate(tapered) on this inside which has to put out straight, passing the transom. The space (niche) which you have created this way, you can fill with thickend epoxy.

That's the normal way to make an edge. You can even make this as sharp as a knife.

For the gudgeon I remember you that Hobie has complete one-piece gudgeon plates on the market for the upper and lower pin together.

At last I would consider not to make holes with bolt-thread in your aluminium plate, but to mount and fix the bolts with laminate and epoxy reversed under the laminate. So that the threaded part sticks out for mounting the gudgeonplate. ( Be sure that they cannot turn if you snug the nut on the outside)
In that way you have completely sealed off the transom.