Congrats! You made a very, very good choice. The 10-core and Vega 64 will serve you well.
I know what you mean about throwing in more upgrades. When I was going through my decision-making process on what to order, I started with 10-cores, Vega 64, 64GB RAM, and 1TB SSD. Then I decided to go with a 2TB SSD, then I himmed-and-hawed about the RAM and finally went with 128GB of RAM...and then I finally said to heck with it: if I have such a premium machine, why not have more ultra-premium storage? I ended up with a fully-loaded iMac Pro. And now that I finally upgraded to 10.13.6, all of the software bugs that were plaguing me are gone. I'm plowing through work projects now like a hot knife through butter.
Congrats and enjoy your new machine!
P.S. No need to worry about single-core performance. Your 10-core will do great! I have 18-cores and I do not feel or experience any perceptible hit in the single-core department...