A "decent preamp" is pretty easy (and cheap) to DIY also, but if you're satisfied with the sound as is, then you don't need it.
All of the "special cello pickups" of which I'm aware are designed to stick on an actual acoustic cello and get some of the body tone. Can't see that doing much good (on its own, anyway) on this solid-bodied axe.
Me, I'd want hot humbucking pairs of magnetic coils. That's a slightly more complicated DIY project, but not really too bad.
Did you make the neck and fretboard yourself? Is it neck-through or set? Any pics of the process?