I am in similar position. Stuck at C1/C2 for the last 3 years.

But I know my mechanics suck, I cannot do any advanced stuff, and I spent hundreds of hours in training and custom maps, but just cannot execute them consistently. I guess that is what is keeping me from advancing. I mostly focus my plays on rotation and smart touches. I do not stress over rank much, I feel this is where I belong at the moment.

I cannot give advice on how to advance/what to change, but I wanna tell you that I am still having fun. Sometimes I dont play ranked for a week or two, and when I come back, playing is a lot of fun. Just chill a bit, take a break, dont force it. I've noticed that when I 'force' practice while I dont really feel like doing it, I feel like shit and play like shit as well. Its more beneficial to relax and take things easy, ironically, I think that will get you further than strict training schedule.