3/4" still sounds a little thin but maybe it'll work if you use epoxy and several layers of glass on top and bottom. At least it would crack before it failed completely! If it was mine, I'd use 2 pieces of coosa covered with glass, but I lean towards overkill on most stuff involving my boat, especially where I've had a failure! Your proposed 50% increase thickness will reduce max stress by a little over 3X. 2 pieces of Coosa will reduce max stress by 36X! The amount of deflection due to geometry at the end of that cantilever beam/pulpit will be reduced similarly. Deflections are also affected by the modulus/stiffness of the material, and I suspect Coosa is a lot stiffer than teak! Where you cut the slot for the bracket, don't leave square corners in the hole because that's a big stress riser. Nature likes a nice radius there as well as along the top and bottom edges of the slot to reduce stress concentrations caused by sharp corners/edges.
Couldn't find my old post I did on anchor bracket install and deck reinforcement, as it looks like search function only goes back 5 years, so here's a link to my
photobucket album with some pics of that project. Anchor flukes are a little close to bow, so I have to slowly lower it to vertical before dropping it, but it works ok if you're careful. New anchor wenches have to be trained however! I used 2 layers of 1/4 or 3/16" plywood coated w/epoxy and wedged up under deck. The thin layers conformed better to the crown of the deck than one thick piece. I've since rotated the center cleat 90 degrees and changed the chain lock from a pin to the adjustable hook like you have. Good luck with the repair!