No need to go elsewere to get an answer that reads: "those Grady transoms take on lots of water and likely need replacement, just did three in the last two weeks.....bulkheads too .... "
on and on and on and on.
--------------------------------------------------------------------
You say the drill corings seem fine? What do you mean?
Wet wood feels cool, dry wood has no temperature sensation to the touch. To practice, take a dry piece of plywood, and drill it, feel the remains ?
And no water ran out otherwise I expect you would say so.
On inner transom to start, unscrew any wire tie loops or clips, and other screws that hold things onto the transom, any water come out or on screws or bolts?
If not continue -
Clearly mark the inner transom at 8 inch square grid pattern, drill 1/8 inch holes 3/4 the depth, don't do through outer glass. Catch the remains in hand and wipe out of drill bit, f, compare to above test. If it is wet, it will feel cool, if also rotted, will feel cool and look very dark or black.
If water gets into the transom, it can't dry out by itslef so no dry rot. It is either wet or not.
If you are a DIY, easy job, if all is well, you can fill holes with thickened epoxy in a syringe (West sell them). If you are not a DIY, don't be drilling hoes in boat, hire it out.
If you find wet and dry areas, report back. Draw a diagram.
There is no better test than taking core samples.
Also inspect transom tie ins (connections) for any cracks, all around. That means, wherever the transom attaches to other parts of the boat back there.
Good luck.