You got it right, bolt thrust is chamber pressure X cartridge head area. And yes, higher pressure cartridges really load the bolt lugs, if only for a short time. And most of the recoil comes from the ejecta (the mass of the burnt powder), not the weight of the bullet. So even if a cartridge has higher energy, if it operates at lower pressure, the ejecta are traveling slower when they hit the stationary atmosphere, so there is less recoil (remember, 1/2 mass X velocity squared!!)
I read your other thread with some interest, having just gone through a similar decision process with a 1893 Mauser. I wanted to choose a cartridge I could convert to that would not run the risk of some ijit overloading it at some future unknown time. I considered the 450 Marlin, 444 Marlin, 405 Winchester, and of course all the X57mm cartridges. Finally decided on 7.62X39, cheap to shoot and enough oomph to down a deer at modest ranges. It's all chambered and feeds, goes bang, ejects, etc. I finally figured out I need an elk thumper like I need a bloody nose.