Based on personal experience, HEPA isn't even necessary when you're filtering that much air.

My house downwind of a freeway and a wilderness area, lots of nasty black tire dust mixed with allergens. It did not come with A/C. I had HEPA air purifiers around the house that I would leave in auto mode and they would run for a few hours a day based on the air quality.

Then I got central A/C installed with a standard 1" MERV 10 rated filter on the intake. Just running the blower for 10 minutes every hour is enough to keep the air quality in every room below the level where the air purifiers would kick on. I've given most of them away.

HEPA is less important in most scenarios than "something that on a few passes will catch everything", which MERV-13 will do. Not all systems can handle the pressure drop of a MERV-13 but many can, and there are some pretty low-drop options out there if you search for them.

Had Claude build a Home Assistant integration for my Rheem furnace which exposes commanded ECM CFM, actual CFM, fan RPM, static pressure (among other things) via Bluetooth Low Energy.

We don't have central air in our new home and man oh man does it get dustier. I did not appreciate the power of those cardboard filters.

This is not a good choice for everyone. Before greatly increasing the load on you HVAC blower motor, make sure you first understand where it is, how old it is, and how inconvenienced you would be if it burned out.

There is at least one big downside to running the fan 24/7: you lose out on humidity control.

Annoyingly, both Ecobee and (at least historically) Nest make controlling the fan independent of the heating and cooling hard. You have to enable either heating or cooling before being allowed to enable the fan. And you can't specify how long you want to run it for, but have to select one of a few options.