I just added a new thru hull for a salt rinse on the anchor
pulpit. Mine was a bit thicker but not to matter. Use a bigger area 3/4" Teak or if you must Epoxied Plywood backing plate to spread the load. I think you will be fine with the countersink, just remove only as much as you need to flush it. At 1/4", still the head of the machine screw is flush with the hull and compressed in the countersink, and that load distributed over the back plate so I can''t see it pulling through or failing.