a website with good suggestions.
https://www.stitchingthenightaway.co...stitch-pieces/
This link says to lay the quilt top onto the batting "right side down". That can't be right. It also says to cut the batt the same size as the top before you stitch them together. I think that's asking for trouble.
The work you show is beautiful. I once made afghan squares separated by a thin line of black. It is stunning and I enjoy it every day as it is always on my bed as a bedspread sometimes used as an extra blanket. It is made of wool, so is very warm and very heavy.
You will certainly need some kind of sashing, and it really should be the same color throughout, so I was thinking about a beautiful green or blue, probably in tone on tone. But consider the thin black lines. Lay it out and test it to see how you like it.