There isn’t a field for this in the project or editor settings, but Godot should already be handling it for you. Since 4.1 the snap options are remembered at the project level rather than per scene, so whatever you set last is meant to carry over to scenes you open after that.
So what you’re running into shouldn’t be happening. I’m wondering if the scenes you keep reopening already have their own snap values saved from the first time they were opened, and those wouldn’t pick up your new setting afterward.
Could you set the grid step once, then make a brand new scene and see what it opens at? If a new scene picks up your value and the existing ones don’t, that tells us a lot. I’ll test it on my end as well.