>Now I wish I knew which files to save for the Hyper-V VM in case I need to go through this again.
Files in folder C:\ProgramData\Microsoft\Windows\Hyper-V are those to keep in safe.
Change Hyper-V Virtual Machines Default Folder in Windows 10 | Tutorials (tenforums.com)
You can use Hyper-V export and import functions. (Export copy files to selected folder/disk, not move.)
How to Import and Export Hyper-V VMs with PowerShell (adamtheautomator.com)