Would really appreciate any views on going to full strength veg nutes and switching to 20/4 lighting now, or wait until the stretch phase has finished?

up a bit, you are looking fine and dandy, bro. When they stop growing vertically, you can stop the veg nutes and switch up to flower nutes, so they can fatten up for ya:dance2:
One Love and Peace to you
I am not sure what you are doing with the EC meter and that is typically for folks growing in Hydro setups.