I perceive that this problem may be veering into the realm of "over thinking"....
And I have also 'been to that movie before...'
No matter what kind of cast head the thru hull fitting has on it, that head and the oozed out (thickened) epoxy will all get ground off - flat with the hull surface.
(I keep a container of the West 404 thickener on hand for all this stuff.)
After grinding, the exterior will only show a round hole where the threaded part goes in.
Minimal resistance to water flow, and approx. .001 extra nano-knot of speed gained !!
Note: this type of install only works with the Marelon firing. Do NOT do this with bronze!