The citric is marinating in the system now. Boy, does it take a *lot* of flushes to get the system fully changed with just the radiator drain! I ran nine cycles before getting clear water.
I'll do a couple of heat-up cool-down runs with the citric before draining and going after the thermostat. Fortunately, citric is far more environmentally benign than ethylene glycol, and I can just dump the block drains straight into the gutter.
The two front thermostat bolts have been Kroiled. I'll dose them again with Kroil every time I warm up the engine. As for the one in back, I'll have to take my chances -- can't see where to squirt the penetrant, and it's too expensive to just blast in there blindly.