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

I'm not aware of any solution that exists to connect a single USB device to multiple hosts simultaneously.

You need a 5th PI in front of the SSD providing a network file service.

Networked storage but that won't be as fast as native drives.

I tend to disagree. It will depend on the network and the storage but it is not uncommon for network storage to actually be faster than local flash card.

At home I have couple of older laptops my kids use for various things. I have installed linux on them and configured to boot over wifi and it actually sped up these machines significantly. I would not advise for everybody to do this, though. The setup is complex (there is no built in support in any distro and you are pretty much on your own) and you need excellent wifi coverage (I have no less than 5 5GHz access points to cover the area densely).

One thing flash card will be difficult to beat is random small accesses if the only thing you care about is latency and not throughput.

It's a gigabit network adapter. Ignoring the extra CPU overhead, latency, etc. that alone limits storage to less than SATA 1 speeds. UASP drives on the USB 3 port can push near SATA 3 speeds with lower latency and less CPU overhead.