self level or thick adhesive?

[Rookery]

I'd definitely level it rather than trying to fix the tiles using varying bed thickness of adhesive. I'd use Mapei Renovation Screed or Weber Floorflex.

 

[Stocks Tiles]

Certainly level the floor, if you want/need the job then just quote for the levelling compound and no labour to do it, the price increase wouldn't be a huge amount - in fact you would be spending as much on adhesive to pack it up for a flat finish. Good finish = good job done = happy customer = recommendations for further work 🙂