Just a follow-up now after a bit more than 1000km. I'm quite impressed with this frame. it seems to be built like a tank, but the comfort is great (of course aided by 35mm wide tires). It seems fairly quick on tarmac as well. Nothing creaks or rattles, so far so good! The gap between frame and headset cap I'll sort when the bearings need replacing.

And a huge shoutout to Peter who has been an absolute pleasure to deal with, answering my many many questions. There was an issue with the first frame I received, but Peter had a new frame painted and sent to me ASAP, no questions asked and no charge. I can highly recommend buying from Peter. The CS-GR01 frame has also been great so far. 

Looks great-what width are the Pirellis?   

Since you're using Shimano, you can get a bit crisper shifting and save a little weight with a direct mount UDH hanger-I got this one with the 1.0mm thread pitch and the fit was perfect. 

https://www.aliexpress.us/item/3256807539398396.html?spm=a2g0o.order_list.order_list_main.49.24f01802AeFswW&gatewayAdapt=glo2usa

They are 35mm. Thanks for the udh tip!