Only front wheels replace at 6mm rear and Dot No. 0512?

Hello, just arrived in the forum because I have a question about new tyres. I also tried the search but didn’t really find it. First of all, we have a Renault Scenic built in 2012 with the following tyres: Continental 205/55 R17 and have 6.5mm at the front and 6mm at the rear on the summer tyres. Now it’s so the front right side of the tyre is a crack and we have to replace this tire. That at least. r, but I’m not sure if it makes sense to replace all 4 in terms of age, profile depth and safety with too much difference in the profile depth between front and back or if it’s enough to replace only the front one this year. And then next/next summer also the (now)behind. We are little riders with yearly under 10tkm, if that’s also interesting. Oh and as a tire I had the Vredestein Sportrac 5XL in mind, maybe also jema can And say something to them. Thank you very much in advance!