When I first started trying the marinade on here and my own experiments, I considered that they were too thick. Over time, I have thinned the marinades down to help overcome what you have described.
I use cheap yoghurt rather than the thick Greek yoghurt but also use half as much and add double quantities of lemon juice and also lime juice. This thins the marinade out really well.
For cooking, I always cook tikka in a hot oven (250c). Cooking small pieces of meat in a very high heat cooks meat very quickly and leaves it juicy and succulent. The hotter the better, so a Tandoor will give great results.
The only time I have experienced tikka being tough or not to spec, is when it is lighted precooked and then reheated with as microwave before service. Yes I know, I was just as disappointed as well, especially cooking it fresh, doesn't take long.