Stephen wrote:

I am thinking I could do the whole room in 20mm thick boards and the
2mm drop from the 22mm boards on the landing would not be noticeable.
I am unsure whether I could "patch" the room with 20mm boards
alongside the 22m ones without it being noticeable. 2mm doesn't sound
a lot and it might be hidden by whatever I cover the floor with
(currently undecided) but I am not sure. Particularly in a room where
you will be barefoot.

Put hardboard over the floorboards.

Bill