I have looked into this before and in doing research on it, I learned that most (if not all) car owner insurance policies covers people borrowing your car (particularly if they do not live at the same address). At least in the US.
From what I can tell, you having non-owner insurance is just a nice boost to assuage any fears that your friends might have about you borrowing their car, being at fault for an accident, and having the victim come after them, the car owner. It’s a weird system.
Where I landed was that it probably isn’t worth the ~$500.
Can’t speak to your umbrella liability policy, though. Given that you can sometimes pay less overall by bundling more (seriously, insurance is weird), it might be worth it in your case to take on the non-owner insurance and keep your existing umbrella liability.