I have tried 10mm allen: too small, spins freely, with very little grip.
Hub brand: Joytech
On the outer side it has 9 ball bearings.
Video showing right/drive side:
Video showing left/non-drive side:
It is a low-end (BSO-ish maybe), 170USD hybrid bicycle, bought from Decathlon. 8 gears; Bicycle model: B'Twin Riverside 120; Derailleur model: B'Twin.
I'm not a bicycle expert, this is my first freehub removal attempt. I'm not sure how this freehub is secured to the hub; any details are greatly appeciated.