I fully agree with your comments that a darker rear flash will bring out the splash.
Unfortunately, the rear flash i am using is a Sigma EF DG 530 with only an ability to flash in High or Low.
Even with the radio trigger (Cactus V6) that is sitting on, it can only control the flash firing and not the adjustability of the Sigma flash gun.
I had a T-Towel over the umbrella to reduce the light somewhat for the pic. I can't move it further back as my home studio room is too small as well.
The flash at the rear and pointing toward the object is a Cactus RF60 which I have full control over at 1/3 stops.
Without a second flash gun which has more control, this is the best I could achieve for now.
I aim to buy more cactus RF60 units soon to match the one I own already - or a Pentax or Godox flash