slow cooking chicken thigh in a spice mix

There is almost no such thing as overcooking chicken thighs. Chicken thighs are simply the most forgiving piece of meat known to man. The only thing that concerns me about your plan is that you plan to dice the chicken. There is nothing wrong with that, but diced chicken thighs are very slightly less forgiving than whole chicken thighs. If you do dice them, you should be aware of them, don't leave them for hours on end.

You will be able to tell at what point you've reached the point of diminishing returns (referring to taste) just by tasting frequently once the thighs are safely "done". What I mean by that is that the thighs have reached a safe temperature (165F/74C). How long that will take will depend on a lot of factors, not the least of which is the setting you choose for the crockpot. Wait for a few minutes after the thighs lose their illusion of translucency. Once they look opaque, they're pretty close to being safely done. At that point, if you continue to cook them at whatever setting you have chosen for just a few more minutes (maybe 25 minutes more on low, 10 minutes on high), you will almost certainly be safe to taste the dish.

Especially if you're relatively new to cooking, I recommend backing up that visual method with a thermometer. It's a little harder to get an accurate reading on diced chicken than on whole chicken pieces, but it is totally doable as long as the pieces aren't tiny. Figure at least an inch square, roughly. Just stick an instant read thermometer into an individual piece or two, and also just in the center of the mass of food (not hitting the bottom of the crockpot). If all of those readings are reasonably similar, all of the readings are at at or above 165F, and the chicken passes the "opaque" test, then you can start tasting.

Cooking chicken thighs in a crockpot is one of the easiest ways to make chicken, and it works great. You can leave whole chicken thighs in a crockpot on low all day. On high, you might want to take them out in four or five hours (ish). If you debone and cut the thighs into approximately 1 inch cubes, then figure about 50% of the time that you would use for whole thighs.

If you Google "chicken thighs crockpot recipe", and "diced chicken thighs crockpot recipe", you will find many recipes that you can use as guides.

If I were making chicken thighs tonight with a spice blend or sauce, I would certainly consider a crockpot for the task.

One small caveat is that you want to use less water in a crockpot than the instructions say for stovetop preparation. You can always add a little more water for the final several minutes of cooking if you want the dish to be more liquid, but you barely need additional moisture at all to cook chicken thighs in a crockpot. I would actually consider using the spice blend like a rub, and then just adding a few teaspoons of water to the crockpot. One of the golden rules of cooking is that it's a whole lot easier to add more (of anything) than to take it away.

As far as the bonus, your ideas sound good. So do peas or potatoes. That question is more subjective than most we see here, so I'll reiterate the Google suggestion. Let the recipes you find and answers here be a guide, but decisions of taste are ultimately yours to make.

For the parampara packets, I think slow-cooking at a lower heat is better, but they are designed to be easy and quick, so I think they give the instructions that will have a meal on the plate as fast as possible, and as the packet says, you don't need to add anything at all, not even oil or salt.

You could slow cook the thighs in the sauce whole, then when they are done, cube the cooked chicken.

Butter Chicken is actually a recipe that usually doesn't have recognizable vegetable bits in it. You may want to look online for recipes for Vegetable Makhani (or makhanwala) for the vegetable version of this to get an idea of what Indians pair with that sauce. Another popular alternative to chicken in the same sauce is paneer.

BTW, you will also want to think about how well the veggies you choose will hold up in a slow cooker and how you like to eat them. This is definitely an opinion question, but in my opinion, root vegetables, pulses and beans do well with long cooking, as do mushrooms.

My personal feelings on cauliflower are that it gets mushy if it is cooked too long in a sauce and not as appetizing. If I were to add cauliflower to this, I would add it close to the end, or steam it separately and add it completely at the end. But that might just be me. My preference for cauliflower is to cook it "dry" (let it steam covered in a pan, in its own liquid) or lightly steam it and put any sauce over it at serving. In either case, I like it still to be a bit crisp, not soggy and limp.