Best next upgrade?

You caught me. It’s on the whiteboard on the shop wall, but forgot to list it here :)
I figured discs in the front at least. I’m torn about BBP upgrade given the rear that also needs upgrade and the cascade $ effects on wheels / tires / etc
go BBP and don't think twice. using stock components it's a robust system that's relatively inexpensive, has huge parts availability and is straight forward to install. you can go with something a little more bling-y and aftermarket too if that's what you're after. additionally there are other great aftermarket components that compliment the system-- tubular upper control arms, shocks, swaybars.

two great benefits to upgrading to BBP is disc brake fitment and it opens to the door to a vast selection of wheels. you can always sell your SBP ralleys!

drums out back are fine for a cruiser, you don't need to go discs out of the gate and can always upgrade later down the line.