External website or customize theme - user auth and session data?

[Erkn]

Hello. I am working on a project for a company as an intern and I'm quite stuck.

What I want to do is to retrieve information from my Shopify store’s database. Specifically customers information about total spent, orders (what they bought), marketing choice

I have a store already. But I want to add an extra page where customers get certain rewards. Say they could unlock certain achievements or get an email with a discount, based on the amount spent and other factors as well.

As well as unlocking avatars/new profile pictures.

I suppose I want to use Hydrogen to make a highly customizable storefront for this? I have one big challenge which is the user's data. I need to make use of the Storefront API. But how would I authenticate the user and handle Shopify session token handling?

Or is it better to try and implement this using javascript/css/html inside the Shop itself?

My vision is having a link on the Shopify website that redirects the user to an external hosted website, or even better if you can embed the website inside Shopify in some way.

I have been struggling a lot to find out which direction I should go and would appreciate clarity in this project.

Thanks in advance!