I've done a few UVI projects myself. The problem I found with wearables is their orientation to the sun changes the UVI reading enough to make them useless. Pretty much you have to put it on the top of your hat, like you show yourself in one of your photos. Based on feedback, the most 'useful' one I did was a tiny fob that just blinks out the UVI on a very bright green LED. I was able to get the BOM cost down below $5 in small quantities by using a UVA LED as the photosensor, vs the Roitner Technik GUVA-S12, which is about $5 by itself.

If you could make it a $5 retail item that costs $1 or $2 to build, you might have a decent product that could sell in tourist shops in beach towns. Surely moms with little children might buy one while checking out with the suntan lotion. Or perhaps convince a skincare manufacturer to use it as a promo or pack-in item. I haven't had time to pursue that yet.

Here's an old blog of my experiments: https://www.matrixwide.com/wp/