And why not 9 grids and using the numerical pad like fighting games notation (example soulcalibur notation) :
G7 G8 G9
G4 G5 G6
G1 G2 G3
It also opens diagonal ranges. The middle grid would be the Grid 5
Oh, also I was looking for tabletop rpg fantasy maps and I found something that MIGHT give you an idea for you grid system on this store page :
https://www.drivethrurpg.com/product/256706/City-Modular-MapTiles-COTBS
Near the middle of the page, there is an animation of a modular arena, on twitter you said something about shrinking arena so that might be interesting!