HVAC engineer here. I'd be interested to know if you've done any measurements with outside air coming through standard F5 filters. This is the typical setup for most ducted AC I come across. Much lower pressure and cheaper than HEPAs. It would be good to know what air quality results as a baseline and also in conjunction with in room purification if you have any data.

Edit: I'm in Australia. Here's a comparison of filter types for different locations: https://www.filtermakers.com.au/selection-guide/