There is no hard and fast rule for order of studying but somehow I feel you need to have a personalized order of studying various subjects as per what you would feel easy or hard.
I can give you some tips if you want you can use them see if there is any difference. If its becoming difficult for you, then you are always welcome to follow your own method which I welcome the most or some other given by some teacher.
Among the 19 subjects,
1.Based Volume
just divide them in short, mid and long subjects
Short subjects :
It’s good to keep few of them in the starting as the matter is less and you will know from where questions and what type of questions can be asked.
These also give you confidence that : YES I CAN DO IT.