First, you can stick whatever you want on the wall and also build up your pre-wall element as you like.
If there is only little space on the left and right of the pre-wall element, smooth the entire wall up to the height of the pre-wall element. For that size, I would then use 20mm Wediplates or similar. You can tile directly on that (have it done).
If you don't want tiles, use moisture-resistant gypsum boards 2*12.5mm, smooth the joints with filler, prime and paint with suitable paint.
We had a similar case and had some 1.20m parquet boards left over. We clad the pre-wall element with Wediplates and glued the parquet vertically and used aluminum profiles on the corners. That looks nice too.
Directly behind the washbasin I would also tile or install an aluminum-Dibond or PVC panel. Something more exotic would be to apply Tadelakt – but that's a hassle and accordingly expensive and rather suitable for those without children.