Tyre opinions

So I'm currently running 245/30/20 Conti sport contact 6s on all corners.

I planned to swap out to Michilin Pilot Sport 4S's in 255/35/20.

The conundrum I have now though is that I have a few mm left on the fronts. The back driver side has about 2mm but the back passenger side is less than 1mm. (To the wear indicator)

Before anyone asks the driver side tyres were replaced and two new ones put on the front so the back passenger is the oldest tyre. There is uneven wear the insides on more worn than the outsides, but wasn't going to get the alignment checked / done till new tyres anyway.

So seeing as there is just one tyre that needs replacing at the moment what would your recommendations be? I was hoping the contis would last till another track session.

Should I get a cheap 245/30/20 which isn't really ideal. Or would it be best to just get 2 new ones for the rear burn through the fronts before getting another set of two new ones.

Then I'd be running 245/30/20 on the front and 255/35/20 on the rear in a front wheel drive, would it be better to do it the other way around?

Thanks!