[devodave]

Here’s an option. It sounds like you can take the boards down, so if you have a thickness planer, you could make a sled for it (1/2” or 3/4” ply, maybe even some 1x2 strips on the bottom to stiffen it). Sacrifice 1 board, rip it into 2 pieces, so you have both a single tongue piece and a groove piece. Plane these down to a centerline of tongue/groove to face thickness just shy of where you want your finished thickness to be. You could also make these two pieces from scratch as they are going to be your register jigs. Mount one of these to the sled, ensuring that your fasteners are well below the lower set point of the cutter head. Mount your T&G board into the mounted register, slide the other register jig into the T&G board and fasten to the sled, this one will be removed and reinstalled for each T&G. Set the depth of the thickness planer to your final thickness, allowing for the sled and the register jigs. This will allow each board to be planed to a uniform thickness relative to the T&G.