Wider box squats can really point out poor hip flexibility. Touching the box is also a good way of getting depth if unsure. For strength you should be engaging your posterior chain. This means hinging at the hips first. Make sure you push your knees out and try and spread the floor with your feet. This engages your glutes and keeps you solid. Also on the way back up lead with your chest first and keep your head back. This can stop with caving. I would also say lower the weight and go for depth first, those added inches will feel like 10-15 kg. I have been trying to nail my power lift squats for over a year now and only just starting to come together with all the cues.

It's always best to just live by the fact that lighter with better form leads to better long term weight gains