@soulcrash I just installed a Velux KIG 300 Gateway and added it via the HomeKit Controller integration to HomeAssistant. Same issue here: I can fully close or open the shutters, but I cannot stop them inbetween. Did you find a solution?
EDIT:
I found a position slider, but I did not yet figure out, how to access it via automation.
EDIT 2: there is an easy solution for the slider and the automation: