If you’re not used to using regular design tools, I recommend using a spreadsheet like a low-res bitmap to make a map. It’s as simple or complex as you want it to be, short of making a 3d or writing a real application—- from painting cells with fill colors to scripting.

If you’re looking for something more visual, have a dedicated video card, and don’t mind going down a bit of a rabbit hole, do it unreal engine with the bazillion existing free assets. It’s one of the simpler 3D environments to learn to navigate and you can just drag and drop stuff from the free assets. I don’t know that I’ve seen another complex application that’s embraced the “batteries included” concept any more thoroughly. Of course, you can go as deep as you want, but I’ll bet you could get very pretty version up and running in an afternoon.

A buddy of mine is working on an app for this: https://www.flourish.garden/

I’ve thought about this for a few years. Sq ft gardening would lend itself very well to a grid like this one. I wonder if anyone will find anything.

IRL? I'd recommend paper.js, it has out of box support for pen.js for mapping 2D objects on planar surfaces.