Using non-shopify accounts to fetch order history

So essentially we have an existing service (offers bedtime stories, sleep sounds etc).

We have our own account system, all passwords are hashed so I can’t re-generate new Shopify accounts with accurate existing user passwords.

We’re adding a “store” section to the app in order to sell sleep related other products (sleep masks, melatonin supplements etc) using the Shopify Mobile Buy SDK. I’d like our “Shopify Customers” this to be tied into our existing accounts system. I’ve got guest checkout working with Apple Pay but I’d like to build out an account section within the iOS app whereby people can see their order history / edit recent orders etc.

I’ve been looking for a way to get some kind of user identifier from which I can fetch historic orders, but all I can find is fetching the “customer” object via an accessToken, and it “looks like” the access token requires a customer to be created which only accepts a username and password as inputs.

I had considered passing the email from ApplePay and a new “token” we generate on our system as the password just so I can “log them in” in the background, but I figured this would impact any future web-store (unless all built custom to use the same token system - which is a possibility), but wasn’t sure if side-things like any emails shopify sends would also be impacted by this in terms of sending them to a web store where they’re unable to login.

Has anyone dealt with how to access order history when trying to use it alongside a separate user accounts system / is there a better way of doing it than what I’m proposing as it all feels exceedingly messy.

Many thanks!