If you change to McNaughtan’s gas struts you can (in fact you must) specify the pressure in Newtons that the strut be pressured to. You buy the strut un-pressured and they pump it to your specified pressure before they hand it over.
Initially you may have to guess a value that may turn out not quite correct (you can increase the pressure, but not decrease it once it’s pumped), but once you get it right you’ll get pretty good service from those struts. Not that they’re rust-proof however.
There’s a thread on this forum somewhere that I started with a procedure to safely compress a Conqueror’s roof struts (750N) in order to fit them. They are a little compressed even when the roof is fully up. You compress the strut with a farm jack and hold it that way with a clamp made from aluminium flat bar and 8mm threaded rod. You slack the nuts off to let it extend to the right length once you have the first bolt in its hole. Easy, safe, and cheap.