there is no quick fix here. its a pain to lift them as you know but the only real solution.
1:50 is a minimum for a softwood deck in my opinion.
90% of people dont even bother to run a string line over to check the joists are all flat and then they screw boards down creating little troughs over a low joist etc.
Yeah I started putting them down when I realised some joists were lower or warped so luckily picked up on that one in time and put it right. It's just the slope at one end that's not draining fast enough and I know I'm going to have to lift a lot of boards to sort that now since, as you say, there doesn't seem to be a quick fix.
Can it wait a few months or will the damage progress quickly? What with short days, cold and Xmas, I might look a bit of an idiot decking in December!