There are no boots best for shinbang... searchbar buddy... You ski backseat, ski correctly = bye bye shinbang.
Dude, you aren't getting it. What happens when you have a softer flex is you are going to have to bend deeper into the boot to transfer energy into your turns. You are going to have to end up putting the same amount of pressure into the tongue to get the performance from the boot. If you have a stiffer flex, you get more from less movement. This also plays a role when paired with your bend ability in your ankle and your arch under foot.
If you are DEAD SET on a soft boot, go try a pair of rental boots before you buy = I promise you will feel unsafe almost. Freestyle boots are softer, try the SPK, but like most people will tell you that it is a park only boot and is crap otherwise (not from opinion, just from heresay).
This is all outside of the fact that you ski backseat. sorry, I'm beating a dead horse cause you obviously don't want to be told this... but its going to be the truth.
learnt to stop landing backseat and it helped temendously, but it didnt totally prevent it. got a footbed and no more shinbang, because my heel was lifting in my boot.
if you have pain in your feet/shins, dont just ignore it. Take them to your local shop, theres a lot they can do for you if they know what they are doing.