"But even a 1kEur GPSDO might not work" and "Path irregularities will be an issue but this can only be overcome with a communication mode which can handle it." - these are exactly the points I want to emphasize.
TELE Greenland has some 8 GHz radios links at 10 Gb/s over 50-60 km but uses 8 x 56 MHz.
The Si5351A accuracy and stability is as good as its clock which might be anything from a sloppy crystal to an optical clock.