I share your pain!! When we had an indoor arena, I swapped several springs out for M90's and now that we're heading back outdoors I swapped them all back...
I have three crazy little letters that will make your blood boil if you've ever watched in envy as other guys with expensive gear play the sport: truth is, none of the above issues exists on the design of the... PTW
I picked up an A&K clone PTW and although of course it's not up to the performance of the Systema, it is a mid-range AEG priced like a mid-range AEG but with the features you envy from the Systema. I am now selling off all of my frustrating AEGs so I don't have to deal with their issues anymore. Now I can deal with the A&K's issues of course, but... no more SPROING every time I open it up
For what it's worth though, I find the M4 gearboxes to be worse than the AKs, and if they are cheaply built ones, the trigger is just as finicky as the anti-reversal latch so you've got yet another thing to hold down.
By far the easiest AEG gearbox I've worked on was the P90, since a bunch of the workings are outside the gearbox shell. So fast to pull the gearbox out of the gun too!
Best of luck, my friend