Before I could turn this:
I had to replace these:
The previous blades got pretty beat up when re-flattening the old planing slab. It turns out that water-based wood filler > carbide blades. But I must have known this day was coming, because I had a spare set of blades.
So after a morning of flattening one face of each board (by hand), then an hour or so of thicknessing (by machine) and smoothing for final glue up (by hand), I had eight boards at a total depth of 10.5″. Exactly half of the total depth on the new workbench.
The glue-up was a nightmare, though. Despite some aggressive shaking, the older PVA glue was starting to get viscous and kept clogging in the nozzle. I am hoping the glue holds up, as it was over two years old. It would be a pain to redo the glue-up, but I do own a heat gun if worse comes to worst.
I can add two more boards to the core and still be narrow enough to pass through my thickness planer. Which I will do next weekend.