Adler, this is all very interesting... Now I'm waiting for you to make nice animation for all the ships which are missing

And hence it is an epic game, I cannot have that many upgrades. How many techs would I need to make 3 modern upgrades for the frigates?
As it's an epic game, I need to make some compromise. I already think that perhaps the WWII line is to complex.
I try to make the best of what is already existing. I cannot add as many units as you suggest, because there are to my knowledge no animation available.
A WWI BB has an A/D of 35/30 and +2 HP, with a bombardment of 12
A WWI destroyer has an A/D of 12/10, no HP bonus, and no bombardment.
So if the destroyer attacks, it has 12/(12+30) = 28% chance of damaging the BB. With a base HP of 4, it means 4 HP for the DD, 6 for the BB...
Chance of winning low... But a lucky torpedo is still possible
If the BB attacks, the chance of damaging are now 35/(35+10) = 78%.
It seems to me the DD is unlikely to win the confrontation.
If we follow your patern, and make a destroyer with 5 A/D and a BB with 40 A/D (your lowest settings), the destroyer has only 5/45 = 10% chance to damage the BB.
I'm afraid it would render the DD virtually useless. In that case, I wonder why one would built any DD. Doesn't it lead to a completly unbalanced game?