Question on Merge Field for Import

I had this same issue. You cannot merge on a formula field. I also found out the hard way. My solution was to make another field:

[Merge Field Scalar]

You simply need to fill this merge field scalar with the formula field, and of course make the scalar field unique. You can fill it via a nightly pipeline that runs a table to table import, or dynamic form rules, or a pipeline, etc. If you use webhooks that can also work.

Essentially, you need a webhook or pipeline that says, if one of my 2 fields that make up the formula field changes, then change the scalar field to the value in the formula field.

Then merge onto your scalar field.