In that case, isolation should be possible.
It's a bit confusing there are two separate Amazon links we seem to be talking about. A green board and a red board. On the red board (IRF540), it seems that on the outputs, all minuses are separate from each other and separate from GND on the input side. On the green board (60N03 mosfets), the minuses on the output side all connect to each other and I think also to the GND on the input side. On both board, all GND's on the input side connect to each other.
Neither of the boards uses LR7843 as a switching element, so apparently you've got a third board which I cannot see the PCB of and I have no schematic of it obviously.
So the above is all second-guessing based on indirect information; your measurements can only tell the story the way it really is.