A few industrial class GPUs would be a good choice, though it might be a good idea to wait until the Ada Lovelace cards come out. The A6000 is still an Ampere card, and probably wouldn't be a big change from his 3090s, other than the available memory. The problem is, that would just encourage him to make his renders even MORE complex, because 48GB of vram would give him pretty much unlimited complexity in any shot. And the A6000 isn't MUCH more processing power than a 3090, and I believe run at lower clock speeds. They definitely built for a higher duty cycle, though, which at least in principle would be a good idea for a developer.