Studding will do the job, but personally not a huge fan of that method, because it takes up valuable space and adds weight, often where you don't want it. Also it won't be as stiff as the box section method I mentioned.
A piston tank will offer little advantage aside from speeding up the diving time- peristaltics can be a bit slow, thus they lend themselves best to small volume systems unless you build a large pump.
One disadvantage of a piston tank, if you use a single tank, is a shifting c.g. the error might only be small e.g. one or two degrees of positional angle out, which a leveller can often deal with, but that's still enough to affect the running characterisitics of your sub. To put it into simple terms it's a bit like trying to drive a car that pulls to one side a bit, you can probably drive it okay with careful attention to the wheel, but it keeps you busy.
A pump system won't give you that problem unless you have a unbaffled hard tank or you allow the bag to move about inside.
In terms of accuracy, the system you have is very good.