Since I didn’t have a 50 mm tire on hand, I taped a 50 mm PVC pipe to the rim for clearance testing. The pipe has an outer diameter of 50.8 mm, and the maximum height, measured from the rim to the topmost point of the pipe where it was taped, was 47.5 mm.
I tried checking clearance with hex keys. A 4 mm hex key wouldn’t fit at either the chainstay or seatstay. A 3 mm hex key fit without rubbing, so real tire clearance would be quite tight at full volume.
As for the seatpost:
The minimum insert is marked at 10 cm, measured on the rear side of the post.
With the seatpost inserted to that point, the maximum saddle height (from the top of the top tube to the saddle rails) is about 23 cm.
Without cutting the seatpost, the minimum saddle height is around 15.5 cm (rails to top tube).
If you cut the seatpost, you could reduce the minimum height further to about 13 cm from the top tube to the saddle rails.
Hoep that helps.
Regarding the cranks:
I tested the FC-R8100 with a 46/36T chainring setup. There was less than 1 mm of clearance between the 36T ring and the chainstay, and while I could fits in a 4 mm hex key, between bb and crank.
There's definitely no way to fit the road cranks safely with a T47 external bottom bracket.
Also ordered a Shimano FC-RX610 2x12 crankset in 165 mm length and a GRX FD-RX825 front derailleur today. Hoping this setup will work with the wider chainline and save me from any more experiments.
I had already been looking at some alternative Ali cranksets, but with the RX610 at €105 and the front derailleur at €100, I’d rather keep things moving and avoid waiting around for other parts.
