Thanks for the answers on here, so here's my final build:
decided on this bike for it has basically the same geo as my winspace slc2.0, just with higher stack, which means I can run it slammed
Groupset: GRX DI2 11spd
Wheels: DT Swiss PR1600 road wheels (had them lying around, will probably be replaced by carbon ones in the future)
Tyres: Schwalbe Thunder Burt 2.1" -> measure 48mm on these 18mm internals rims, 4-5mm clearance all round
Casette: 11-42 XT
Chainring: Alugear 46t – has 1.5mm of clearance to the chain stay on standard GRX chainset, so standard chainline.
Stem: Deda Zero
Handlebars: FSA Energy Super Compact 400mm (380 at the hoods)
headset: used the Ritchey Logic-E internal expander and top cap as ICAN didn't want to send one; the top bearing sits pretty low in the frame, so a standard Aliexpress top cap didn't fit.
bottle cages: to come: TI from Aliexpress
replaced most of the absurdly heavy mounting bolts (40g) with lighter ones (12g)
on a positive note: the brake mounts were faced by ICAN, that was unexpected.
Shipping was very fast, frameset arrived 4.5 weeks after ordering (had to be sent from China and finished first, as was out of stock in EU)
Payed 785€ for the frameset (Debby sent me a 80,- discount), got the GRX DI2 used for 630€, so in total the build was well below 2000€ if I factor in the wheels and small bits and bobs. pretty happy with that.