We’re in the same boat (literally same niche and vanity widths). We decided we’re going to install it anyways and see if the gap drives us nuts.

If it does then we’re going to buy custom side splashes extra thick with the right cut outs to make it look full/continuous.

Other ideas we considered:

• adding drywall on one or both sides to narrow the niche ( was going to be a pain because the electrical would need to move)

• creating some kind of hidden 2” storage pull out to fill the space

Such a pain!