I like that you've got your own thing going on there matey. FWIW I think you're right in that the JCR clear has a lot more 'stuff' going on - the cazpar will not be that bass even if you change out electrics etc. Different scale and materials like you say.
My experience of custom build is that it's great in theory but you don't necessarily get exactly what you need at the end of it. Very difficult to translate your feelings and inner thoughts into a clear spec that delivers the outcomes you want. This said there are great builders out there who will no doubt do a fantastic job. Alan or Martin at Sei, John Shuker etc.
I guess it starts to get really hard to find used basses to try once you go below 33 (and that's hard enough to find to begin with).
If you love the JCR perhaps you need to work with it. I'm guessing the challenge is the voicings but let's face it, depending on how ambitious you wanna get there will always be challenging voicings even at 30 scale!!
Good luck with your quest!