Don't stress too much about it. Easy mode needed at most 3 historicals last event for E3/E4 (don't remember) iirc. And those 3 include all the other ship classes aside from DDs! For example, E4 needed only 2 historical in Nishi.

If you're really concerned about the DDs, just level a couple of ones you have currently to kai (Asashimo for Kurita, Nowaki or Hamakaze for Suzuki, shiranui or pick up any of the other ones at 1-1 for Shima).

If I were you, I'd actually level the other heavier ships to Kai in the historical fleets, such as Nachi to kai, Ashigara, or imo more importantly chiyoda/chitose to k2.

And I dunno if you have tried this already, but you can level DDs as escort ships when you're doing 3-2a grinding (I usually throw 2 DDs in as escort).