Thanks to both of you, this is indeed valuable input!
Currently I run the Panaracer Gravelking 28mm, before Conti GP 4 Season, and I really like them. I think they wear down faster then the Conti, but I guess that's a tradeoff. So will try for sure the larger version.
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sallen73: Does the clearance is due to your fenders? Soma says with fenders max. 45mm? I guess I'll look for the largest fender which fit.
Thanks also for the tip with the mechanical brakes, this could be one option if I go for the Alfine the bar end or Gevenalle shifter route.
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chas58: for the new Shimano GX series are in-line hydraulic second brake levers existing, I'll have a look at that. Since the Alfine with brifters is easiest with Di2, all brake levers I see are actually for hydraulic brakes... I really have to calculate the actual price difference of my different options. And yes, the Tamland looks nice, but no dropouts for IGHs...