Choosing filler for spa removal

I just removed a below-ground spa and the accompanying wood deck in my backyard. This has left me with an approximately 5' round 1' deep hole where the spa used to live, as well as a dozen or so holes from the deck's 4x4 posts which were set in concrete. I would like to level this area and lay some St. Augustine to match the rest of my backyard.
What is the proper filler material?
I'm worried about settling, and ending up with low spots. Since it looks like a yard will do the job, I'd rather just get whatever works best (i.e., saving a *little* money on filler by using cheaper material is not attractive if it will result in more work or leveling down the road).
I hope I've given enough information, and I would appreciate any suggestions out there. By the way, location is Houston, TX, if that matters.
Thanks!
Tom
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