Typically this should be a simple process.
Like you, my core game is installed to the default on C:, which is a 1TB NVMe.
I had my content installed to a 2TB SATA SSD at D:\MSFS.
Then I decided to add a second 1TB NVMe to my system for MSFS so my stuff would load faster. Once installed and formatted, this new drive became F:. I copied the entire d:\MSFS to F:\MSFS.
Then I opened Windows Disk Management, and swapped the drive letters, making my new NVMe my D:, and old SATA SSD F:.
That was all that was required. Everything worked like a charm.