I know that these bolts are good, we use this type all the time for floor fixing.
I've never used them through wood though, but I can't see why there would be a problem.
I would have said try and get at least 50mm into the wall, but you can achieve that with the 120mm bolts if you countersink the hole slightly ( drill out the size of the head) and that will hide the head too.
Why Thunderbolts ? Can you not remove the posts after drilling? I would tend to use a plug into the wall for this application, but if you have to laeve the post in place then use the thunderbolts, I'm definitely not a fan of frame fixings for this type of installation, they tend to come loose.
I assume that you are fixing the posts all the way up their length?