UTMs unreliable, best way to track partner referral

[simonbarker]

We are in the process of setting up a partnership with a large and much more established company. They will refer their customers to us, and we will offer those customers a customised experience.

To do this we need to know for 100% certainty when they send us a customer. The obvious solution here is a simple UTM link, however my testing (and subsequent googling) shows this is very unreliable on Shopify. Calling the graphql end point and requesting the customer journey summary shows that for many orders the ready status toggles to true and there just isn't first visit or last visit UTM data (this is reflected on the order page under conversion details).

A second option is an automatically applied discount code but that's not ideal since the couponing apps like Honey will find it and then all our customers will look like they have come from the partner (this will also conflict with further partners we bring on)

Next up is a dedicated landing page, and reading this from the customer journey summary. This appears more  reliable than UTMs but still not perfect looking at orders in our production store.

We could use a dedicated product for the partner but this feels a little silly

Could a subdomain like `partner.ourdomain.com` be the most reliable way? 

To provide the custom experience I need to be able to detect that they came from the partner and then have our server set a metafield on the order so a third party app isn't really an option I don't think.

Any guidance would be great. I'm pretty stumped on the best approach here.