Simply replacing the 3090's with 4090's would provide a major performance uplift assuming your model fits. (I have rented both 3090 and 4090 systems online for research, this comment is based on my personal experience, it is well worth the price increase and the hourly rate for the inference speed you get)

I am not a lawer, but shouldn't 4090s be worse since they don't have nvlink?

there are patched drivers for enabling p2p but if I remember correctly, they are still slower than having an nvlink