Let me get this straight, you can get full wall panels up there but not 4 parts of a prefab unit

To eliminate a lot of seams that could collect mold, and may need caulk, I was thinking a prefab would be one unit. I've seen some prefab ones but wasn't thrilled about the seams.

Agreed, a prefab surround is a thril-less endeavor everytime. Your cashflow will dictate if you can ramp it up into a waterproof solid surface installatiion.

Corian would be great, as would marble slabs. But I have a feeling you're looking for a more modest solution. Kohler has a new product called Choreograph which is a panel system. It's a step up from a premolded unit!

Have you looked at the Maax Utile option?