It all depends on how much backwards compatibility you require. You can use your sites statistics to figure out how many customers/visitors you’ll be loosing when you disable TLSv1.0 and TLSv1.1. If that’s acceptable, it would be best to disable those indeed.
And why did you think the grade B was being caused by the Let’s Encrypt certificate? Did you think a Let’s Encrypt was less secure or something? Why do people still think that? Sigh…