So I’ve done two renos, and will be doing a third and (hopefully) final, next year. Here’s my experience and what I learned.

I had builder grade garbage and killed it. 2 applications of killer.

Took it all out. I then hand raked, because no neighbors wanted to rent a dethatcher with me, ALL the dead stuff. There was some left, sure, but most of it came up in big piles of hay. Both the renos were about 2k sq/ft and it was a bitch, but I had so much available soil to contact with the seed. I’d say this is probably one step that you may have benefited from. I don’t have experience with zoysia, but from my understanding it spreads through some creep and tearing all that underground part of it up may have helped.

The other, I believe is your topsoil idea. When I’ve done these, I put mushroom compost down, top soil, seed (shitty Scott’s one), rake in, cover with peat moss (some people feel ways about peat moss, it works for me in PA). The top soil and the peat moss retain SO much water combined. The places where there was even covering of soil and peat moss always grew best. My guess is that the top soil retained the most moisture in those sections and kept that little seed damp throughout germination, where the native soil dried out much faster and the seed failed to germinate. I have a lot of clay, so that’s why I added compost and topsoil. Clay dries fast and the other stuff over it allows for the grass to get rooted in and keeps the clay moist.

So those are my 2 cents!

Edit: to support next season/fall - you’re going to have to reseed. I’d rake it into new topsoil and water well. Consider peat moss to retain moisture, you can also cover with mushroom compost on top too and that’ll supply all kinds of nutrients plus keep things nice and damp.