Hi Paul,
Have you got the little plastic tube bit, just before the depower toggle? That stops the toggle getting pulled all the way up to the side of the butter box. Its available as a spare & if you don't have it get one or just make one from some rubber tube!
If depower (on any kite) is hard to pull through try the following:
1/ push the bar out to take the load off the front lines...
2/ stop edging so hard to take the load off the front lines...
still stuck?
3/ sheet out & bare away to totally unload the front lines...
"y'cannae break the laws of physics, Jim" so work with them instead.
The depower is more than effective on the rally IMHO. You have most of it on the bar throw - enough to pretty much shut the kite off. Even with the trim line all the way out, so on full power, if you let go of the bar with the stopper at the top the kite just flaps fully depowered?? The trim line wont give you more depower than that! It just sets where you hold the bar whilst riding for a comfortable position/level of power.
Regarding 12 & 8 as a quiver, this is what I've had last season & this year again. 12 works for me @ 76kg in a range between 10-26ish knots & the 8 between 17-35+ knots. I'm using a skim (or sector from 8knots) in light wind & the high end ranges quoted are not averages they are the gusts you can still kite in on a TT without issues. When choosing a kite to fly it's either windy or its not - so its a simple choice.
For me, I think you should work on your edging skills & get comfortable flying the Rallys in the top half of their wind range - where you start to get some real performance out of them
Dan