Would I get too much temp swing? Just can't see putting pex under the wood on main floor as having all that much impact. Am I wrong? Am considering as I am doing solar hot water anyway.

Radiant tubing under wood can still supply a lot of heat.

We put radiant pex in the basement, forced air upstairs. Waterfurnace makes a geothermal unit that does both hot water and forced air. We love it. Basement stays a very steady 74 all winter long and we feel actually helps heat the main floor. The hot water feature of the furnace also let us put pex radiant under the bathroom floor and in a few other places upstairs.

We are heating and cooling 5000 sq. ft. in Illinois for about $800 of electricity per year. ICF walls, r54 in attic. 3 ton geothermal unit