I would agree with all that has been said. I'm already trying to lesson the variations in total thickness with mahogany wood shims (I have a home wood shop, I like to build furniture for a hobby)and thickened epoxy.
The idea of the trim ring is as follows: With the existing frame there is still .200" of gap when the frame is fully tightened, so we are relying on sealant to fill that gap, and .200" is a lot of gap. We are replying on tightening the inner frame to provide clamping pressure so the outside frame is sealed against the deck side wall. The smaller we make the gap the less movement, the less total reliance on a sealant. Remember, when the frame is completely clamped down, with no sealant there is a .200" air gap. If we lessened that gap to say 50 thousandths ( .050") I think that we will see a better long term outcome. Picture it, Inside wall, trim ring, inner frame. Now the inner frame is pushing on the trim ring as we tighten the screws, that has to be a better situation then before, even if there is variations in wall thickness. Besides, you will get a cute little teak halo around the fixed ports... How's that from trying to convince you? Your smiling right? I am...
Did I explain it any better, still have doubts?