You're going to love riding that beautiful bike. I had the pleasure of helping build up a Lynskey GR250 for a friend who I ride gravel roads with fairly often. He loves his bike. He says it really smooths out the bumpy roads.
He also went with mechanical Ultegra with hydraulic discs. He's running a 11-36 cassette which works perfectly with the Ultegra mid-cage RD. He went with a White Industries R30 crank. Initially he ran 32/48 rings, but later he switched to 28/46. He also uses that same Easton EC70 handlebar. We've put those on a couple of custom gravel bikes for customers and they both are happy with them.
He runs Panaracer Gravel King SK 35mm tires tubeless. On his 25mm inner width rims, they measure 40mm. There are lots of good gravel tires around nowdays. You can probably get better advice on what works for the terrain near you from local riders. My preference is to go big and definitely go tubeless.
I just saw this about a possible way to get 30/46 chainrings on your Ultegra crank.
https://www.bikerumor.com/2018/01/09...ct-chainrings/