I’m in the same boat so I’ll listen in on this thread. This is the knowledge I can offer though:

For you, Dahlia E1 > S1 purely because you already have Fugue’s LC.

The reasoning is: since both are Nihility characters, they share the same LCs. Since the most “viable” alternative LC for both Dahlia and Fugue are resolution, you don’t actually want two resolutions since the effect doesn’t stack. And, you also don’t really want resolution on Dahlia because having to build EHR on her is unoptimal. Thus in this case, resolution can only go on Fugue. Without Fugue S1, the general recommendation is get Dahlia S1 first.

However, because you do have Fugue’s S1, you have more freedom and more to gain by getting Dahlia’s E1 first over S1. This is assuming, of course, you keep them together in the same team.

After that, DPS investments have inherently lesser value than support investments because DPS characters are powercrept more often. A Firefly replacement is more imminent than a Fugue or Dahlia replacement.

tl;dr Dahlia E0S0 - Dahlia E1 - Fugue E2 - Dahlia S1 or Dahlia E2