custom fitted wetsuits - any experience here?

some of your problems would be addressed by moving away from a spine zipper wetsuit, such as your water flow while finning.     The semi drys tend to zip from shoulder to shoulder, like the dry suits.

And the vest add on is the easiest of all - could even go with your existing.   The custom suit I mentioned was a two piece- 7mm farmer john and then a 7mm vest with a dry suit neck seal.   That covers up the zipper, adds insulation, and removes any water flow at the neck.

However, like any neck seal, it's a bit tight, esp out of water.  And getting into the vest is tough - I eventually had to have a zipper added on one side as my once broken shoulder doesn't flex as well as it used to.   This won't be as severe with a thinner, stretchier choice.  You need the water flow deterrent more than pure insulation.

If it is hard to get into, you may have to do it on land, and then endure the sun/heat on the ride out to the diver site.  The hooded vest under the main suit generally works best, but doesn't mesh as well with the day boats' ride.

Your rationale against the dry suit is incomplete.  If you think you're peeing because you're cold, then it's the obvious answer.  You won't be as cold.   A trilam drysuit does not compress at depth - it's just as warm as it is at 20ft or 140ft.  That was why I got one in 2000.   At 140ft, a double 7mm suit is pretty thin.

I eventually stopped diving with it as I shifted mostly to warmer waters, and shore dives in CA involve a long surface swim, so it can be > 2 hours from start to end, and my suit had no zipper or plumbing.

The trilam suit will also dry quite fast, so you wouldn't have to change suits for the final day of diving.  And it would let you adjust for different temps by wearing different clothing underneath.