I use 1/2 rennet tablet to 18 liters of milk , 1/2 tablet = 1/2 teaspoon of liquid rennet.
As I understand , there is approximately 60-90 drops in a teaspoon , and you only needed 1/2 teaspoon for 18 liters.
It looks to me like you used about 2- 3 times as much rennet as you should have.
Excess rennet cause rubbery curds , which is what you have , so it seems likely.
Calcium shouldn't be needed for raw milk , I doubt that had anything to do with it.
Next time measure at 1/8 teaspoon per gallon of milk approximately , that is what I use ,. and it works for me.
That said , I've also had way better and more consistent results using the tablets instead of liquid.