Can you use points from an external system for purchases in Shopify?

[ReINFaTe]

Hi. For context, I'm very new to Shopify development.
We have an app with our backend and a Shopify store.
We have a points system on our backend and we would like to set up a system where users can use their points for Shopify purchases (either pay full price with points or partially).
Not sure how possible it will be with Functions API, but unfortunately Shopify Plus is not an option for us at the moment.
Another idea is to create discount codes per user with Admin API.
I would like to know if there is a better way to implement it without access to Shopify Plus features and also an opinion on the "discount codes" idea.
How possible it with Functions API will also be good to know