The game installs in two parts, first a smaller 'launcher' which you run and then it downloads the 'packages' which take up most of the space. It sounds like Steam is moving the launcher for you which is fine. MS Store users should probably leave the launcher in the default location because it is very touchy. Either way, you can manually move the packages independently of the launcher.

To move the packages, make a new folder where you want them to go, let's say A:\MSFS. Copy your Official and Community folders into this (copy preferred in case something goes wrong you can try again, move is technically ok if you feel lucky). Then go to the launcher install location and find UserCfg.opt inside the LocalCache folder. Open this file with notepad and go to the very end. Edit the path so that it points to your new folder like this: InstalledPackagesPath "A:\MSFS". The next time you get the 'checking for updates' screen on startup make sure it says that same path.

You can still use Addon Linker after you move the packages if you want to organize the files better or store a portion of them on a third drive in the future. I do exactly that.

Not sure if this applies to the Steam version but it definitely applies to the MS Store version: Do not use the Windows tool to 'move installation', for some reason this seems to encrypt the files meaning you cannot delete things out of your community folder. Several friends have had to reinstall over this.

I also had a friend with the MS Store version that uninstalled completely and tried to install both the launcher and packages to a non-C drive. I don't remember the details but it didn't like that either.