My suggestions, skip the corrosion resistance, the factory coatings are pretty decent, and will probably last longer than you own the trailer. My trailer was a 2007, lived in Colorado for about 8 years; then Arizona and California, original gas springs out in the weather and mud and no noticeable corrosion.
I waffled on the reducible force, but ultimately decided that calculating my best estimate of high and low forces then picking the mid range and hoping for the best was in my favor with respect to cost, buying a second pair seemed better than reducing too far... but I only needed 2. Also, the odds or reducing them all the same without some sort of balancing tool seem really low.
I looked up some SAE documents that seemed to indicate a best practice of designing SUV hatches for 16 lb opening and 20-24 lb closing forces. I assumed I’m a bit stronger than a soccer mom (that depends on the day) and have myself a 30 lb open and 50 lb closing (I do have weight on my side in closing) window. It worked out okay, but I made a mistake in mounting, and without some more bracket fab and welding I’ve sort of limited my opening distance.
With respect to the annex, I think you will find that 4” low isn’t the worst thing in the world. Ground is rarely perfect anyway. I set mine up so my wife and kids should generally be able to walk under it, but I can’t quite, so it is like 68” off the ground in most situations. It seems about right, but sometimes the annex rumples a bit at the bottom. I know I got the height right because it is right above my sight line, and I hit my head on it a lot during building!
All things considered, for a straight-up lift, I would look to linear actuators... but I think a high cost tolerance is helpful there.
