Id put 6mm down , just so i don't have to wear my PPE Everytime i go for a shower [emoji41]i can't see the thermal thingamabobs being that much different in a little bathroom,
I thought that the minimum thickness for overlaying a timber (chipboard or t&g) floor was 10mm if using the insulation type tile-backer boards (Wedi, Marmox and etc.), so I would recommend checking the manufacturer's spec to make sure before proceeding.
The floor would need to be deflection free regardless. The thickness of insulation board only makes a difference to the insulation property value.
10mm I prefer on concrete floors but with timber 6mm is fine.
But regardless the floor needs to be deflection free.