I also have one loop of modern baseboard. Fortunately, it's in the attic conversion where they did insulate the rafters while doing the conversion, so it works even at that lower temp. I did do something slightly unconventional in plumbing that zone in a primary/secondary and it gets the water from the boiler "first" and returns it to the primary loop ahead of the main zone which is all cast iron rads. That means the baseboard gets the hottest water possible and the full potential flow from the boiler if it "needs" it. In practice, that zone tends to only run 4-5 hours per day while the main zone is running 22-24 hours, so either what I did works really well and/or I didn't need to do it in the first place.

But, you've already discovered your reset curve with modern equipment, so you know the right answer for your place.

Thanks for the story on mercury pressurization! Fascinating. I learned a lot about our old house (originally gravity circulated as well, but near as I can tell, pressurized only to the typical 12-15 psi and with an in-ceiling green steel expansion tank: https://structuretech.com/wp-content/uploads/2020/03/Old-sch... )

(And of course, sorry to hear about your contamination inconvenience and expense!)