Just when you think you've read every thread about arcpy/Pro to get your script to run and feel confident to have mastered this ridiculous convoluted world, you're back to square zero. I mean I have literally spent hours reading about installation/configuration of Python 3.x/conda with ArcGIS Pro. I've replied to a number of threads feeling knowledgeable enough to chime in. But I don't know what's going on here.
I have an SDE maintenance script running on one server under a service (domain) account, and it works beautifully. I had to:
Install Pro (this is 2.3).
Create a Portal user for the service account (for licensing) and make it admin.
Open Pro on server once and connect to correct portal.
Upgrade to latest version of arcgis package. No other changes to conda env.