First, I try to keep FSX working in the ever-changing / evolving Windows 10 environment! Remember when the Microsoft slogan, "where do you want to go today?", was changed to, "where in the hell are you taking me today?".
To your point, I am limited to FSX SP1 due to Windows 10, so I concentrate on creating a geographical area (Seattle) and duplicate all the real-world commercial traffic in and out of KSEA, KBFI, KTCM, and KPDX. I started by adding the Alaska / Horizon schedule(s) and am now adding other U.S. airlines.