Based on your information the NiftyKeyz will send multiple melodies so you are sorted on that point, even without the mult. So either passive or buffered will work in your case. It also depends on whether you have enough power spots in the case to power all modules including the mult.
I don't think the decay out envelope on the LPG will be all that useful, since it's decay only, not even Attack/Decay. You probably want more complex movement in the filter and also outside it than just decay.
I think I would remove either the Frap Tools 321 or the Erica Synths Black CV Tools, they are kind of in the same ball park. And then use that space for another envelope. If you plan to play the keys a lot rather than just sequencing, you may want to add an ADSR for perfect control. You can then double it to the filter or use another envelope.
For filters, Erica Synths Black Multimode Filter, ThreeTom Modular Steve's MS-22, Tiptop Audio Forbidden Planet or System80 860 (MK1/MK2) are all good choices with the same or lower HP as the LPG at a somewhat similar (secondhand) price. They all have very different flavours.