The average 4 person house uses 190 gallons per day. About 4 of that is drinking water. So my wastewater contains about 2% more PFAS than a household not using an RO system. Assuming that all of it ends up in my body and not in the toilet.

Yes, I'm agreeing with you that you're doing the only remotely sensible thing. My calculation was considering only alternative disposal options for the RO wastewater, not your entire household water usage. (1 gallon of drinking water per person seems like an over-estimate as well, making your point ever so slightly stronger.)