Hi @Wade , You can just extend the length of a long token in ArcGIS Server settings. There are some pros and cons of using this approach, but it is pretty easy to configure. This article has all the details (oldie but a goodie):ArcGIS for Server Long Tokens (spatialtimes.com) . I wrote that many years ago, but I just logged into an ArcGIS Server 10.9.1 instance and the setting is still there to modify.
As for the proxy approach, there are lots of examples out there for different languages (.NET, PHP, Java, Node.js, etc). This will depend on your server platform and what you feel comfortable with so hard to give a relevant example.
Both methods above can work with ArcGIS Server standalone. If you have access to Portal, Esri ArcGIS Server help pages do explain how to leverage Portal/AGOL for accessing secure services (with credentials saved in the item): Connect to secure services—ArcGIS Server | Documentation for ArcGIS Enterprise.