Depends on how fast the “2nd PC as a render node” can render.
1 Gbps = 125 MB/s
10 Gbps = 1250 MB/s
100 Gbps = 12500 MB/s
M.2 NVMe (980 Pro local) = 7000 MB/s
Latency will be much higher in any Network environment vs. local M.2, but latency will not be relevant to the task of rendering output to file.
The bottleneck will primarily be the CPU cores hence the dependency of “how fast” is your render node at the CPU level … in my experience “render to file” (or stream) on a very fast PC can generate about 300 MB/s I/O load.
Without knowing much about your “2nd PC”, I would still suggest that 10 Gbps should be sufficient I/O performance.
You’ll need 10Gbps LAN cards, 10Gbps switch, Cat 6 or higher cable, and your cable run length is limited to about 50m (164ft).
Yes, getting a 3090 “at retail” price is very difficult, took me 11 months to get ONE EVGA 3090 and I signed up on their waiting listing the day they became available. I refuse to pay scalper prices and even some of the pre-built system markups.
I’m currently running one Titan RTX 24GB on a AMD 5950X and it’s MUCH faster than my backup render PC (7900X with Titan X (pascal)).