You will find this very useful
https://forums.flightsimulator.com/t/complete-redownload-of-main-files-when-updates-solved-steam-users/472608
This problem of having to redownload 100 GB of the sim with any update is happening to many Steam users simply because of poor " Layman Terminology “. The sim’s default install is on your “C:” drive (Regardless of where you told steam to put it !) , and you choose not to have such large files located there, ( along with steam and all your other games), People choose in Steam to locate ( Install ), those files to a larger drive somewhere other than " C:”. You must understand that the file location you have chosen is for " Steam’s client only", and not the SIM itself. So many people choose to tell the update installer the file path they have selected for their steam MSFS location. This is NOT CORRECT !. If you want the Large files of the simulator ( For me approx 160GB ), off the C: drive, you must find the Default install folder on C: This is a manila folder called " PACKAGES". This can be found @ C:\users(your name)\AppData\Roaming\Microsoft Flight Simulator. Look here and locate the " Packages", folder. Right click on the folder and " Cut",. Then paste this folder on the Drive you want it on. DO NOT try to put it in your client folder. Allow it to stand alone on the selected drive. For me I pasted it on G:. Now when I look at my G: Drive I see a, “G:\Packages” , file path.
Next time there is an update, simply direct the " Browse" function of the MSFS installer to this file. Note you do not select this folder with a “right click\ select folder”, in the pop up window…rather just left click on the “PACKAGES” , folder , highlighting it and then left click the “select folder”, option located in the bottom right of the opened window for the new installer path. This is going to work flawlessly for anyone who has been experiencing this issue like me since day 1 of the sim’s release like me . Cheers !