PXE Boot w/ NFS root - you won't get the bandwidth (about 112MB/s over gigabit Ethernet), but you'll definitely see the response time improvements (IOPS + latency).
This. I intend to do some more testing with my current cluster to see how much improvement I can get with one Pi 4 serving the traffic (vs each Pi running from its microSD card). And ideally seeing if a faster machine with faster storage could do even better.
If you end up doing this you should definitely make a post about it. I'm sure I'm not the only one incredibly curious on the real world performance and tradeoffs