An idea to sync wishlist – have you considered using Shopify Forms to submit this data?
Form is in Shadow root, but it’s open, meaning you can get into it with JS and push your wishlist data json.
- It can auto-sync submitted data into metafields.
- It can tag customers upon submission, which may trigger a Flow.
- It saves each submission into metaobject, so your data is accessible this way as well.
Removes complexity of setting up a custom app.