baffled by small amount of wort extracted (relative noob)

[jof]

Yep, if you have a separate mash tun, then I would continue sparge until I had the required volume of wort in the boiler.
Its how I do BIAB, mashing in the boiler and running wort into a FV to see volume.
then removing grain bag & putting the wort back into the boiler for the boil.