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Hey mate, your year long research and digital noodling led you to this - GREAT =)
A few thoughts:
FM synthesis and the Castle are an incredible combination. Only the modulation possibility of the four operators is already a mind blow.
Cool as well that you build your own case to start with - for me the case is one of the main investments but while it is required right at the start, it might require completely different things after a while and during your modular development. I for my part started with a 60hp Moog case to compliment my mother32, then moved to a Performance 104HP case, then to a second one then a Make Noise Skiff and finally to Doepfers Monsters. What I want to say is, it took me the hard way but in hindsight I wish I would not have aimed to perfectly fill the case while ever evolving. Great to see your approach here!
If you use the Es3 and linked Ableton CV Tools you have a gazillion source of modulation potential there and you could integrate via ES3 or even think about the ES9...
Mults...I see your point and based on your current idea and set up (quad VCA, LFO, Envelope) it might not be necessary...but then Batumi should stay =) one of my first modules and a stay from day one - so much potential and so many possibilities of usage.
While I am on the quad thing...Zadar...I find myself using it rather as a envelope for Filter modulation than VCA gating...for that I prefer more tweakable and classical AD (for sequencer based triggering) or ADSR (for keyboard based triggering). What I try to say, Zadar is amazing and deep and unique however (for me...) works best in combination with a classical AD/ADSR option...
On the filter side and considering space...maybe Endorphin.es Squawk Dirty...stereo with a VCA!! and a High Pass Filter on board! 8 digital filter emulations. Worth a look...
Regarding the Bloom...soo many alternatives and yet unique and linked to personal preference...have a look into Euclidian Rhythms, Melodicer, Tuesday, Ornament & Crimes or Turing Machines
Complimenting the CV Tools I just say, maybe look into Make Noise Function as a Maths alternative (half the space with both EOR and EOC outputs AND a freeze option!)
When you say does not feel right - what you mean? Maybe an integration of your guitar would do the trick? Like that you would be able to jam out together with the machine... For that you would need an interface given by e.g. Intellijel + 1U Pedal or 1U i/o and maybe think about effects (or you want to do all that in Ableton?