Buy what you miss or don't have! You only know that when you are making music.
I feel GAS from time to time, but I know what exactly I'm after. And even then, I try to find the reason why I want so badly to buy this synth or that DAW!
FM8, Zebra and Aalto are perfect companions
For me, I would add Massive (or other wavetable), Razor, Prism and the Legend. All have different kind of synthesis and can produce unique sounds that not easy to produce with other synths. There is overlap of course, but in general, with the synths you have, that would be my perfect minimal setup.
I have all the above plus other synths like Dune 2, RayBlaster, Nemesis, Largo ... etc minus Zebra but I think I already have what cover it