We don't recommend replacing two tyres at a time. Different tread patterns can destabilise a vehicle. I'd either swap to four all-season tyres, Michelin CrossClimate+ are a good choice, or I'd replace the front tyres with the same tyres you have on the back wheels. Best 'standard' summer tyres for you? I'd probably go with the Goodyear EfficientGrip Performance 2 as they're quiet, refined and good on fuel economy for the high mileage you do. Another choice would be Michelin Primacy 4. I'd go for whichever you can find in the right size at the best value.