Your junction temperature depends on ambient. You can’t just say “I’ve got 100 degree ambient, but I want to dissipate more power”. You can’t “modify the equation” because you just don’t like the results. That IS a realistic estimate, what makes you think it’s not?? What makes you think RthJA is not accurate? If you’re not going to believe the data sheet, who ARE you going to believe, some strangers on the internet?
And 100 deg ambient is REALLY high. What’s your environment, the surface of Mercury?