What if instead of applying the (possibly) necessary increase to the chasis directly, it was applied through gear selection?
If that could be done, making a number of the defensive options equipment based would then the increase interoperability of Pf2 And Sf2 classes.
So something along the lines of Tech armor coming with HP add-ons based on your hit dice, giving a weighted bonus to lower hit die in order to give them the defense they might need for ranged combat, or possibly have some bonuses added to armor types that grant extra defenses against ranged attacks.
There's options in the future to make equipment matter more, and it makes more sense IMO according to the fiction.
It would make some sense from a story perspective, but I think even the influence that gear has on your basic stats in PF2 is way too high. Adding even more to that isn't a good idea.
It would also (slightly) increase the complexity unnecessarily, because you would have an additional factor in your HP calculation, but only on some classes. Then there is the issue that it only solves half the problem, doing nothing about armor proficiency. Lastly, there are the uncomfortable questions that the GM will have to answer. I can practically hear the "why does he get the cool thing and not me" asked by a player with an 8HP class who doesn't care what PF2 does.
In short, I don't think that's a good idea, SF2 itself comes before interoperability. The latter can be achieved simply by giving a short paragraph about it being advised to adjust HP and armor proficiencies when using the class in a different system.