There is no one-fits-for-all installation order for FSX; especially when you have a number of addons to follow. Here is the order I took. Note that my FSX system and addons are stored on separate SSDs. Moreover, I used Acronis Home Image to backup each stage of my installation, just in case I need to go back to any point for any reason.
FSX-03-Scenery Tech Landclass
FSX-04-UTX USA, Canada, Europe and Alaska
FSX-05-GEX USA/Canada and Europe
FSX-06-Real Environment & Active Sky Evolution
FSX-07-Orbx Australia SP3 with mesh and updates
FSX-08-Orbx North West America with updates
FSX-09-Orbx Australia airports with updates
FSX-10-Orbx NA airports with updates
FSX-11-OZx and Ants airports
FSX-12-Must-use Scenery Addons
FSX-13-Current — updating with adding and removing addons
I store all these backups on a networked HDD. Total space consumed so far is about 300GB.