In an ideal world, the solution would just honor the order in the Table of Contents, but as I understand it this is not default/possible as Pop-Ups are displayed in the order of rendering (https://community.esri.com/thread/114482 & https://community.esri.com/thread/182833-specify-order-of-results-from-multiple-layers-in-popup-web-appbuilder).
If this is not possible, I am looking for insight into how I can default a layer to always go last. In practicality, I have three hidden layers in the TOC (Roads, Parcels, Throughfare) that are used for custom pop-ups. But if I turn on 'Schools' and click on a school, it will show me 'Parcel' first and I'll have to click the 'next' arrow to get the school.
Full disclosure : I'm not a GIS developer but have to act as the org's de facto GIS developer since we don't have one