Here's what I do to still use Instant Scenery as a design tool. I keep an installation of FSX on my PC that is only used for creating scenery in with Instant Scenery. Then when it's ready to test, I install it in P3D to check how it works, looks, etc.
Of course having FSX and P3D installed on my system plus all the ORBX scenery regions, FTX Global, etc. can fill up the storage capacity real quick, but that's how I make things work. I do limit what I've installed into FSX. I'll only have one ORBX region or FTX Global installed and active depending on where I'm designing at. For the most part, I've been using this configuration since P3D arrived over five years ago.
Works for me.