Backyard redo

So last summer we had an issue with a sewer pipe and a large section of my yard was dug up to replace a pipe. I leveled it out some and spread grass seed on it and actually got some decent coverage and growth into the fall. I was mainly wanting to avoid it being a muddy mess with kids and a dog playing back there.

Now I'm thinking about nuking it all, or tilling it all up, and starting over. I've got a big rear-tine tiller already, so that part would be easy to do, not having to go buy or rent one. But would that be the best option?

Should I just try to kill everything now and scalp it, or is tilling a better option. There are some high and low spots, but mostly level, at least the back 2/3 of the yard is, with plenty of sunshine. I was thinking of getting a large load of dirt or topsoil delivered and till that into the soil. We do have some clay in the soil, so my thoughts are to get the dirt to till into that and also help with leveling. Is spring time good to start this, or is late summer/early fall better? Our whole lot is around a half acre, and this is just the middle section of the backyard. I'd guess maybe .25 acre at the most. There are some beds with trees and shrubs around the perimeter. I could probably have the whole thing tilled in an hour or two max.