Rubbery chicken is caused by overcooking, and under cooking.
The reasons being, if your using chicken breast, when sealing the pieces, if the pan isn't hot enough it allows the moisture to cook out.
When using chicken thighs, if the searing isn't quick enough again it cooks the moisture out , rather than sealing In the moisture, if the searing is too low it doesn't break down the collagen in the thighs, leaving them "Rubbery".
The sealing of the chicken is No. 1 in the fight against Really Rubbery chicken.
I don't normally use curry kits, I have a vast selection of spices, and three different curry powders, Carribbean, (a light and flavour some) great for prawns,
And two different Indian styles. A dark medium hot for venison, and beef. And a Tandoori powder, which saves me having to mix all the spices myself.
Onions are very essential and when softened I add the spices.
Good luck.