Transferring fluids between barrels is fucked up, in 1.12 you could dump or scoop out a barrels contents into an empty barrel or one with the same fluid, in 1.18 the only thing you can do is right click a barrel with an empty barrel and pick up it's contents.

I seem to remember trying to implement this at one point but not being able to for some reason, but I don't remember why. This seems doable?