I can't help but think that you're still running your boom way too low. I'm 190cm and never run the boom at chin height unless I'm very powered up/overpowered on my 9.5, and buy then it's 22-24 knots or so. I never run the boom at shoulder height, once I'm hiked out in the straps that is way too low to be able to transfer enough weight onto the rig. With my 11 and 12 (and I'm NOT on 11 in 10knots if I can help it) I run the boom somewhere between eye level and forehead level, i.e. a 10-15cm higher than chin. Try running your boom at eye level or so and really concentrate on getting as much of your weight as possible transferred through the rig and mast joint instead of through your feet during pumpung and acceleration.
At your weight (I'm "only" 95) it's extremely important getting your weight onto the rig for planing and acceleration, and that's pretty hard with the boom too low.
Another thing is that not really having great acceleration with a 10.5 for a 100kg rider is not really that weird. If you look at a regatta field in 12knots there's not a whole lot of people on 10.5's in 12 knots, and definitely not anyone at 100kg. I think the main problem may be that you're really underpowered a lot of the time. That may also lead you to rigging your 10.5 (guesswork, I know) for power instead of speed/stability like you normally would a formula sail. I would believe that an 11/11.5 would give you more power and better acceleration, while also allowing you to rig the sail with enough twist/release to have great top-end in 20knots at your weight.
Personally I prefer to stay on my 12 (Vapor) until the wind is at least 14 knots, from 14 solid/even knots my 11 starts to catch up, but not before. The 12 gets going a little earlier, and definitely accelerates better and points a lot higher once planing at those windspeeds.