the right way to do it is to start hacking that thing apart from the inside; chop everything with a 2" flange around the bottom/ sides to glass back into place peel off the inner glass skin and rip everything out. then reassemble & fill
Basically this but on the inside
Your (half-ass) approach probably cant be any weaker than the rotten wood that was there before.
think the most difficult problem is gonna be drilling enough air vent/ weep holes to prevent airpockets - youre basically filling an ant-hill thats non-porus/ relatively sealed with a thickish batter consistency.
P.S. you probably should have started this project several months ago- good luck getting her splashed by august..