I know this doesn't answer your question but if you want minimalist look have you looked at getting a stainless or brushed stainless niche?

Something like this. They come in different sizes too.

https://ap.rdcpix.com/b5d88f54cb201a7346a69318ebf2f270l-m2297722865od-w1024_h768.jpg

I think your location of the niche is perfect to not draw attention to it, however instead of a niche build a ledge instead with all with the same tile as the walls. Something similar to the above link, but ignore the accent tile.

the photo above is to show the tile. Our shower is just 48x36, rendering below. We have a teak bench we bought at Costco that can serve as a seat. The shower is so small I don't want something jutting out.

We have a two cubby ledge I had customized between our studs. Made sure to ask for mitered edge on the schluter which apparently is not an easy task but they did great!

Jessica Basma - love your bathtub! Who makes it? \