Gap between countertop and wall

[Deluks]

So not just tile ? How’s backer board fit in a rapidly narrowing gap ?

Let me go back and reread the original post, it can't be that fekkin hard.

 

[Deluks]

So not just tile ? How’s backer board fit in a rapidly narrowing gap ?

Well it is a **** show, and requires a radical solution. I'd use a 6mm and 12mm cement board, and get creative with the tile adhesive, it would be slow and painful, more of an arts and craft project rather than quickly lashing some tiles up in a hurry.
It's what I've done with uneven and sloping floors.

 

[foxhole]

Better to overboard the entire wall and tile which would hide all the problems . Though OP limited photos don’t shows the entire wall which would help .

 

[RrogerD]

Well it is a **** show, and requires a radical solution. I'd use a 6mm and 12mm cement board, and get creative with the tile adhesive, it would be slow and painful, more of an arts and craft project rather than quickly lashing some tiles up in a hurry.
It's what I've done with uneven and sloping floors.

I fitted a shower into a non-square, non-vertical corner. 45mm down to about 15mm. Had to use packers of differing thicknesses for the full tray length and full shower height, using multiple boards. I then tiled. It was a ball-ache but totally doable and came out fine.
Then did the same for the floor below as well!

 

[Deluks]

I fitted a shower into a non-square, non-vertical corner. 45mm down to about 15mm. Had to use packers of differing thicknesses for the full tray length and full shower height, using multiple boards. I then tiled. It was a ball-ache but totally doable and came out fine.
Then did the same for the floor below as well!

Yes, doable, but it helps if you have the patience for messing about with numbers and angles.