Thanks for taking the time to make this detailed write-up! You got a beautiful result, and I appreciate the spirit of sharing what you learned and helping people who are considering similar projects.

I'm glad you measured CO2. I've recently noticed I tend to get headaches when working or sleeping in small poorly-ventilated spaces, and I'm surprised how often I encounter new structures without ventilation. I guess tightly-sealed buildings are still a relatively new idea.

In the PNW you could probably be comfortable >90% of days with a cracked window, fan, adjustable shades, and a little electric radiator, but if I was doing this project I'd probably opt for the modern heat pump / ERV combo as well.

There isn't really any clinical evidence for CO2 levels impacting stuff beyond so called "sleep efficiency," where the effect is small (above 1000 ppm is 5% reduction in sleep efficiency) and it has the counterintuitive effect that less efficient sleep is sleeping longer, which is usually what people want. That said, placebo effects are still real.