Smoothing material to flatten not level?

[Dumbo]

Ardex feather finish. It's like filler bit pricey and don't know what maximum thickness is

 

[Geoff Stafford]

Sadly I'm not at the location so a photo is not possible.
I estimate the maximum thickness is about 9mm.
I read that Ardex feather finish is not up to this depth.
If is was not a wooden floor ( to be covered with cement insulating board) I would put a mortar mix down and with straight edge timbers I would drag and tamp it to a flat finish.

 

[Flintstone]

If your sticking boards down with tile adhesive anyway just make good the worst areas and stick them down!