It’s a pretty easy change if you build from the source, but I agree that a 5x5 grid is needed for a bed this size. Yes, it slows down starting, but of the overall print time, an extra minute to probe the additional 16 points is worth it.
I don’t want to mess with the firmware, I can do it! But I just wanted to offer that idea to Lulzbot to consider. 5x5 is what I use on my print farm machines with beds this size.
I’m honestly a bit disappointed in Marlin for not supporting “dynamic” bed leveling points. The firmware itself says “max” for the number of points, but there’s no way to do fewer points. My prusaslicer startup GCODE dynamically changes the points on the bed that are probed, so I get a higher resolution matrix for where I do print, but I’d rather use 5x5 for anything over 200mm x 200mm, but drop to 4x4, or 3x3 as suitable instead.
I understand that they want that memory block reserved for the matrix, but there shouldn’t be any reason it can’t build the matrix from only 9 points and interpolate points to generate a 25 point matrix to fill the memory block properly.
Oh well, it’s just a few more seconds at startup.
I know this doesn’t really help since you’re running marlin but there’s a project for klipper that will generate a dynamic bed mesh for only the area that you’re actually printing. Maybe someone will eventually port it over to marlin…