Shimming the neck will alter the angle of the entire neck, not just those frets. So this won't change the relationship between the action at 12 and 24...
You said your bridge is low in the body... just raise the bridge a bit. just over 1mm action is really VERY low... crazy low by most standards.
2mm on the low E string and 1.5mm on the high E at the 14th fret is what Ibanez suggests as 'factory standard' setup.
I keep mine at about 2.5 and 2 low to high, but all guitars are different so it depends, but I never go lower than that.
If you like your action that low, you're going to have to live with some buzz, even with 'perfect' frets it's almost impossible to eliminate buzz in the upper register with the strings that close to the frets, unless you have a really light pick attack, and I mean, just grazing the string with the first couple of molecules of the pick. LOL
If you must have your action that low, straighten the neck to either .1mm relief or try to get it as flat as possible... then raise the bridge so your action is where you want it... this will cause some of the lower frets to buzz, but your upper frets will buzz less since you'll have a bit more clearance in the upper register this way since you'll have to raise the bridge to maintain the action.
I do, however, suggest you try the guitar with slightly higher action. I know it's nice to have the 'buttery' feel, as if you don't even have to put any pressure on the strings, but you'll hear a big difference in your tone if you give the strings room to breath.
And believe it or not, after a short time the guitar won't feel harder to play any more... not only that, but it can actually feel like you have more control since you can feel the strings better than with low action.
I used to like insane (less than 1mm) low action. I injured my left arm when I was a kid and used to have a lot of pain when I played and thought this was a way to relieve that pain... it was more detrimental to my playing than a benefit and I'm glad I was talked into trying a more 'reasonable' action.
Of course, everyone is different. It's my understanding that Joe Satriani likes very low action... and he has NO control problems and good tone... as always YMMV
You can leave your relief right where it is if you do decide to try your action a little higher...