I would start fresh with the existing code as a cheat sheet. That might give you some ideas and a new look at the codebase, maybe you will find something to throw out, something to refactor, and sometimes some laugh (personal experience kicked in here when I was reviewing my own code).
Also - consider using Razor UI class libraries if applicable. Using this approach I was able to get back to "single file deployments" - where all my belongings for the plugin is packed nicely in 1 or 2 files to be installed in the consuming project.
If you need more info about how to pack plugin - https://blog.tech-fellow.net/2021/09/03/how-to-pack-your-shell-module-for-optimizely-content-cloud/
Anyways blog post is coming up describing my journey for the localization provider.
We have a plugin built for CMS 11 that adds a few MVC views in admin GUI and also som views in the edit gui.
Is it possible to upgrade the CMS 11, framework solution or is the best way to start with an empty Core project and add everything again with Core 5.0 references?
I just need to know where to start here.