Large gap between counter and wall (10-30mm)

Hi, I've fitted 18mm ply counter as a substrate I will be microcementing, the front edge of the board is exactly where I want it to be, but the back is against an uneven, raw wall. The wall will be tiled as a backsplash. Options:

1. Cut and fit lots of small blocks over the 2.4m span to close it off to something caukable
2. Fit an equal angle pvc L shape to bridge it (worried this will be too raised for the 3-4mm layer of microcement to cover)
3. Expanding foam, sand then fill /...

Large gap between counter and wall (10-30mm)