Sorry, I'm still learning Flash Print myself, so no expert advice from me.

What about under the Infill Tab, Combine Infill, the Maximum Solid Combine setting description includes "infill will be thicker while thickness of the perimeter keeps unchanged".

Setting that to 2 might do what you want.

I really like using Flash Print more so than other slicers, and calibrated my printer using Flash Print, and not ready to go through that again with another slicer yet; too many things to print now.

So I'm interested where this setting is in Flash Print too.