We are currently on Kentico 12 and are making the switch to 13 MVC Core.
One of our most important and widely used widgets is a custom "Control Wrapper" widget with a "File System Selector" type field, that allows the user to select a File location (via a of an .ascx user control, which then gets loaded on the page.
These user controls get and update external data from our CRM system via a REST API middleware, and display and allow our customers to view and update data. I am new to MVC and am having difficulty coming up with a similar solution, since the concept of user controls doesn't exist in MVC.
Is there a way to do this, or are there any suggestions on how to replicate this functionality in Kentico 13?
welcome to SO!
Migrating from Portal Engine (asp.net web forms) to MVC is not an easy task, so kudos for attempting that project on your own. My suggestion for you would be to take all the User Controls (ascx) and convert those to version 13 Widgets and remove that wrapping "Control Wrapper" widget as it just complicates the whole process.
I'm sure there are some areas you can "globalize" like the overall connection and authentication to the middleware but creating version 13 widgets from your version 12 user controls is the way to go. The documentation is your friend and there quite a few blog posts and video resources as well.