This is very helpful, Heinrich. My local university apparently has a good German program and I have been thinking I might eventually take some courses there to get up to C1 and even C2. Once I'm 65 (which is approaching quickly), I'll be able to take classes without tuition.
Obviously, I will consult with them if/when I am seriously about to do it, but I'm trying to get a sense of what sort of preparation I would need.