So I finally managed to put together the bike and have a first test ride...
Impressions:
Geometry is way different than any other road bike I've been riding so far - quite upright, feels extremely comfy, but I can still fold into a aero position nicely... Fits me like a glove I'd say...
I know I run quite a pile of spacers at the moment - unfortunately need them due to injury but from my ride yesterday I'm confident that I will be taking out about 10 mm by the end of the year...
The bike feels quite snappy but the upright position gives an impression on beeing high up - will take some time to feel as comfortable going around corners at speed... I guess I'll have to learn cornoring in the drops

Surprisingly positive: No toe overlap whatsoever - actually quite some room (even though my shoe size 48 is quite big for my body size at 183cm)...
Only downside: I'm having a bit of a hard time to get the saddle into the right position - not sure if it's also slipping a bit due to the clamp mechanism... will try again and maybe go for a big pile of carbon gripper... (But actually I believe I didn't torque to spec - the build was going over several weeks)
Long story short: Very lovely ride...
Here's the Parts list of the final bike (and afterwards the thoughts of why I've gone down this route):
Frameset: XM Carbon Speed - size 56
Groupset & Brakes: Shimano GRX Di2 1x11 (Brake calipers might be swapped for 4-pot-brakes later)
Handlebar: Pro Vibe Aero - 420 mm
Stem and Headset: Ritchey Comp Switch (also tried the WCS Switch - could get a neater routing with the Comp Switch)
Chainring: Alugear 40 T Golf Dimple Aero Chainring (might be switched to a bigger ring for the future or switched back and forth according to the planned ride)
Chain: YBN 11s SL
Cassette: ZTTO SLR Gen2 11-46
Still unknown: How on earth will I mount my garmin on this stem/bars combination...? and how can I mount the front light - it keeps on slipping backward on this bars...
So there was a quick discussion and several people suggested that this should be 2x as it is a road bike... I still decided to go 1x as i believe this also fits for Aero reasoning (and that thing looks quite aero). My main thought behind it is, that both my daughters are running the same groupset and if anyone of them might encounter a problem and we need quick remedy for their next races my bike could act as a donor... As for cadence: I'm not really sensitive to finding the "perfect" cadence since I just have too much other issues on bikes at the moment, noticed that I switched two gears more often than just one gear at a time yesterday...