I have far fewer buttons on the stick in my Legacy but there are three that I consider essential:

1.  China hat with elevator and aileron (not rudder) trim.  My experience, in contrast to another writer, is that I make aileron trim adjustments immediately after takeoff and thereafter about every 3-5 minutes.  Rudder trim about every 2-3 flights.  For some reason, the effect of P-factor in a Legacy is minimal after about 50kts on t/o roll and nonexistent at climb speed or higher.  Aileron trim, however, varies quite rapidly with fuel use.  It's not a problem to fly out of trim (at least in VMC) but I learned to keep the airplane trimmed in the Navy and I find it really annoying for the a/c to be even a little out of trim.

2.  PTO (obvious)

3.  Autopilot quick disconnect.  Actually, I hardly ever use it but really want to have it if the autopilot is going berserk and I need to keep my hand on the stick.  Put it somewhere where you won't inadvertently toggle off though.

The other thing that I would like to have (but don't) is a speed brake switch because I fly break patterns a lot.  If you don't do breaks, it doesn't matter.

I have the flap switch in a recess in the console and like it fine there.  Don't think I would want that on the stick and really would not want it anywhere where it could be triggered inadvertently.

The radio switch system that someone described sounds interesting.  Again, I would worry about inadvertent use.

Of course, if you plan on guns, rockets, bombs etc...........