That's a long way to cut with a diamond blade in a circular saw. It would take a long time and probably several blades. If you had a cheapo circ saw it would likely be ready for the trash when you were done too.
Best bet is to rent a masonry power saw.
They will cut curves if you get the technique right. I would practice cut in the waste area until I got the hang of it.
This is what you want to rent
Check with rental houses, they will most likely have the Stihl brand for rent. Maybe others as well.