If you're set on filling the gap between the Reality and UC2 then the most likely options are probably an original Rubicon, RST X-1, or Dark Code. But that does seem like a pretty unbalanced bag with three big asymmetrical balls and a Hustle Ink.
If it were me I would look for something somewhat below the UC2. The Phaze II and Zen are solid options. I wouldn't be too concerned with overlap from the Zen and UC2. Their overall strength might be kind of close but their ball motion shapes are not.
The idea of the Phaze II would appeal to me more though. Seems like that would be a good idea for being the daily driver while having clear options for more strength (Reality), more backend (UC2), or stepping down (Hustle INK) available. You could take a look at the Honey Badger Intensity for this slot too if a little more shape is needed.
Going for something stronger than the Phaze II but weaker than the Reality with the Axiom, Zen Master, or upcoming IDOL Helios would work too. Although in that case I'm not sure you wouldn't just be able to use the Reality instead for most situations.
