Castor & Pollux might be difficult because the company is finished making modules, and the used prices for them can be pretty steep.
More importantly, this rack needs modulation. LFOs, sample and hold, random sources, function generators...the more, the merrier. Marbles (chaos) is a good place to start, but this system has ton of sound sources and effects to be modulated, and hitting them with more modulation will make things feel more alive.
It also needs VCAs and/or attenuverters for that modulation or for other routing. Definitely look into the difference to see what would be most useful to you. There's lots of good and interesting small options. The Polivoks one might sound good, and I can't speak for or against it, but smaller and less elaborate VCAs for modulations. It would be worth considering a 3xVCA and 3xMIA or some other VCA/attenuverter situation that works for you.
In terms of the Hive, I do feel like there are other tools that might be more helpful in here than voltage controllable panning, like mutes or even a 4x2 Stereo Matrix Mixer for unique mixing, but you'll know better than me what works best for you in terms of that. How good is the Behringer interface at taking signals from the modular currently? Have you tested this?
I know you said you have a preference for DIY, and that's a good way to go in Eurorack, but it is worth considering that you've sort of painted yourself into a corner by picking a lot of very large sound sources while neglecting modulation and utilities. Now all of the more efficient and feature-packed the modulation and utility modules (digital or analog) that can fit into the space you have left are either too big or not DIY. I don't really have a specific answer to this for you, since my solution would be to get rid of certain things you seem to already have, and I'm not really trying to change your system into something different from what you want. It is worth considering how many other types of module are in here that can make your current modules more flexible and more playable, though. If you had a little more space in here, it would go a long way. Maybe there's room for exploring this in a future iteration of things or a potential expansion.
In the meantime, one potential DIY option to look at is the new Ornament and Crime version that runs 4 apps now instead of 2 and also adds dedicated audio ins and outs. Lots of options to try out there. They say it is a "difficult build", but it might be a fun challenge. The original also has lots of DIY options if you decide that's more chill. Cubusynth makes some interesting kits, and their passive attenuator, quad VCA, and quad VCLFO kits might be of interest to you. Nonlinearcircuits also has some cool stuff, like a Triple Sloths, a triple VCA with lots of features, or their dual LFO/Osc. If you haven't already looked at their website, they have lots of good ideas. I also saw this and thought it might help: https://www.etsy.com/ca/listing/1318579918/froal-quad-function-generator-eurorack
Lots to consider. Too much, probably.