There is quite a lot in this question.. Some points to consider;

1. Which ecommerce platform are you using and just how much information can you pass to klaviyo with each order?
2. If you have 40 niches, how are they arranged with products eg Do you have 10 products specific to Nurse nice? Again depending on eCommerce system you may be able to add the products to a collection eg Nurse collection and then pass that to Klaviyo
3. Think early about possible use cases for this segmentation so that it is set up properly from the start.
4. You may also need to enlist the helo of a developer if your eCommerce-Klaviyo set up cant handle everything you need out of the box.

Some resources here;

https://help.klaviyo.com/hc/en-us/articles/115000250912-Understanding-custom-profile-properties

https://help.klaviyo.com/hc/en-us/articles/115005074627-Profile-properties-reference

@SeanMcC We have specific pages specific niches and we have category as niches coming in Klaviyo
our integration is with Print base

Very interested in the solution of this segmentation