>Ducts are still needed to circulate air

>A recirculating central heating system doesn't do that.

They definitely circulate air, and they definitely filter the air. Whether there is fresh air depends, mostly no.

Good callout. Some people have the strangest reasons to hate high-efficiency ductless systems.

Except it's incorrect. The best system is a "ducted ductless" system that circulates and filters air throughout the house.

I've seen that terminology used on US websites, it took me a while to get my head around. In Australia it would just be called ducted reverse cycle. In the vast majority of cases it would be "ducted ductless" I guess: outside unit connects via refrigerant lines to a heat exchanger and fan in the roof cavity, when then distributes the heated/cooled air to rooms via conventional ducting.

Nobody calls it that, it would just be called a ducted heat pump. The refrigerant lines just run to a central air handler where the ducts start in the house, instead of the air handlers that mount on the walls.

How do you go ductless in a 6000 sq ft home?

Two multi-head units should work just fine.

If it's an existing build with ducts, stick with that. If it's a new build, then like the other poster said, you add more units. See the top of any commercial building for example: heat pumps all over the place.

Another option is to supplement an existing ducted system with a ductless one adding additional zones where you might want conditioning over night. E.g. bedrooms where you want to sleep in cold A/C in the summer where the rest of the house can warm up a bit, or a person in the household who likes their room to be warmer than everyone elses in the winter.

A recirculation system will prevent high levels of CO2 buildup in occupied rooms, particularly with closed doors. I've measured the difference in our bedroom overnight between having the fan on and off, and it was dramatic (if I recall correctly, something like 700ppm in the morning vs 2000). Even if it's only keeping the level consistent throughout the house, that would be a benefit. But every house will have some amount of air leakage as well; a recirculation fan with a fresh air intake (which all modern ones have) will pull in fresh, filtered and conditioned air to replace air that leaks out. If that's not sufficient for indoor air quality, then you can crack a window or set an exhaust fan to run periodically. I haven't generally found that necessary though.

ERV/HRV has its place it less moderate climates where there is significant efficiency loss from pulling in outdoor air, especially if indoor conditions require a significant amount of fresh air turnover. But a central HVAC system with a recirculation fan most definitely can be beneficial in a lot of cases.

It’s worth noting that ASHRAE ventilation standards are based on “bioeffluence” levels which may or may not correlate well to other IAQ values like CO2, VOCs, etc.