Self Directed Coverdell ESA question

I’ve heard a lot about the self directed ESA, but let’s say I was able to save up enough for my child’s college education outside of the ESA (or perhaps they get a scholarship) so I don’t need to touch the ESA. I see that, although I can’t contribute beyond 18 years old, I can keep the account open until the child turns 30 at which point they can transfer it to another beneficiary. I’m guessing I can continue investing it tax free for the child, which could be a fantastic advantage. What happens at 30? Can I only transfer it to another ESA?

Also, are contributions to the ESA supposed to end by the 18th birthday or up to and but not including the year the child turns 18?