I'm certain it's ready. I left most of the basswood I had processed just started in the woods with semi half-assed covering using cherry bark that came off in sheets. It worked fine enough. I processed mine in the winter and I brought it home in fall. I tested it with a MM and it said it was "gooder". It was under 20%
I grabbed a split this afternoon and checked internal MC. Just below 20%. Beat up tarp is tripled up and wood off ground. Single row stack with minimal sun.
I burn it in my gassifier OWB during shoulder times. Does pretty well in that but I don't know that I would be relying on it in a fireplace or standard stove. However I have trouble leaving any wood that is solid to go to waste.
Once someone pointed out that I had a lot of basswood on my property I felt disappointed, but it's actually very good shoulder season wood and seasons very quickly in a matter of months. For early winter my go-to is to start the fire with basswood and then throw a small amount of some maple, ash, or birch in once it gets going just for the coals.