This is making the 3rd rate quite useless. She can't go to the mid tier not to the top. Maybe OW but there the Consties are much better.
I've read many times that 80% of the ships in the big battles were 3rd rates but I guess it won't happen here.
I do undertand what the purpose of this is but I think you should rather come up with a system where there is a cap for each type of ship or maximize the battle rating. I know it's complicated but yea, ppl will always try to bring the biggest ship they can. Wouldn't you?
Or capturing a port would build up from several battles which have ship type limitation. The winning side of these battles would gain important advantages for the main battle like less/more (powerful?) towers for the defenders, better wind, AI help, more spot, whatever. Or simply best of 3. If you win 2 of these battles then the port is yours. (ofc gathering 75 ppl could be problematic for small nations but here comes the diplomacy or the battles should not happen at the same time)