I have some thoughts on subs/destroyers and there interaction.
Right now the problem is 1 DD completely counters an unlimited number of subs, esp with the dreaded DD+Plane raid. But detection roles and the like are a pain. I have two ideas that may work.
1 plane per 1 DD can attack subs.
1 DD may prevent 1 sub from submerging.
Either one of these I feel will allow subs to be more useful. The absolute most important thing, however, is that DDs should not be allowed to block sub movement. Take that single thing away and subs become much, much stronger in the Atlantic.
As far as tech goes I would propose a point system. Assign a point value to each tech. Purchase researchers as normal. Roll as normal for research, but add up the total rolls and apply them toward the selected tech. You could assign different researches to different techs, but cannot change the tech they work on once purchased. Once point value is reached, tech is gained said researchers are lost. For example.
As US I want heavy bombers eventually, they are worth 30 research, so on turn 1 I purchase one heavy bomber researcher. I roll and get a 3. Now I need 27 more research to get heavy bombers. On turn 2 I decide I want super subs, and since that tech isn’t as powerful I only need 15 research to get them. I purchase 2 researchers, both for the subs as I want to sink japan’s navy now. I roll a 2 for my heavy bomber researcher, leaving 25 left, and then roll a 4 and a 5 for my subs, leaving 6 left and with any luck on turn 3 i’ll get my super subs.
This will allow you to slowly work at a tech, rush for it, and choose to go for the good but expensive techs, or take an easier, faster, tech, or all of the above. Now all nations may buy a heavy bomber researcher turn 1, but hey there is nothing wrong with that!