You might have some luck in evening the strop out by pulling it taut at either end and working it up and down over the edge of that marble worktop with a sharp angle in the strop, face side up. It needs an awful lot of doing this to work, but it may help. If you get cupping, just work the edges down with your hand while pulling it taut, immediately before stropping.

If the waves have been caused by skiving the leather so that it is thicker in places, then I don't think you can remedy it. You can check for any significant thickening with a micrometer, but tiny increments are OK - you are looking for 0.5 mm, probably more.

Regards,
Neil