You should use a knocking machine
Not sure what the name is in english, but a machine that knocks at the same time as it turns the screw.
Such are sometimes used when chaning tyres, something I do twice a year.
Prior to attacing you should apply copious amounts of oil or whatever. It always helps, at least somewhat.
If you can, try to apply some heat (you can) and some cold (ice, frozen Nitrogen) in order to get the metall to expand and contract. This should be repeated many times. And oil, did I say that?
With this, I succeeded to remove stainless screws in Alu, that had been out-door for 25 years.
Worth a try.
Best of luck