After doing a little more research, I think I'm going to go with a phased mitigation strategy

1. Leave the HF unit as-is for the time being and exercise discipline with the respirator, even when I'm 'just making one cut'. The HF with a Wynn does a good job with chips and picks up at least some fine dust, and before I had ANY dust collection I could work until there was a dust haze in the shop and not notice it with the respirator in place. However, the HF is just sufficiently low-end that throwing more money at it just isn't a wise investment.
2. Improve overall shop airflow. I'm going to knock a hole in the back wall and vent the swamp cooler through it. I already own all of the bits I need for that installation, so it's basically be 'free' except for labor. It's a house-sized unit, nearly 36" cubed. Put it in the back of the shop and crack the door, and it'll make a big improvement by blowing the dirty air outside for dust, fumes, and everything else. Plus it'll get the fan off the floor where it currently lives and free up shop space. A win all around, and it'll be useful for my follow-on plan of:
3. Eventually replacing the HF unit. I need to do some more research on whether the setup I have will call for stepping all the way up to a full-blown cyclone, or whether a Delta 50-760 or a Grizzly 1029 single-stage will do it. Based on some rough calculations, those single-stagers seem like they have sufficient airflow with the SP drop my current setup imposes. However, that assumes a clean filter, which is hard to maintain. A cyclonic pre-separator would make that less of a problem, but that imposes a SP cost, and that's where the numbers get a little dicey. A clearvue or Onieda Gorilla seem like overkill, but I'll have to do some more research. Having two lungs doesn't mean that one is a spare!