Apologies for the very long delay in following up, but here is what I found:
1) The USB-C cables that came with both the Kingston XS2000 and SanDisk Extreme external SSD's suck. I had problems connecting these drives to my iMac's TB3 ports that were solved by upgrading to Cable Matters 40 Gbps USB4 cables, whicih have the benefit of being reasonably-priced and USB-IF certified.
2) USB-C connections are supposed to be the cat's meow because they are so small, and symmetrical. The symmetry is a nice convenience vs. USB-A. But there's a big downside to them: Because they have much less surface area supporting the same cable than USB-A or HDMI (or even mini-DP), they are much less robust and get loose over time. I found this both with the ports on my 2019 iMac, and with the Cable Matters cables. [It's so loose on my iMac that I'm going to get it repaired under AC+.] That's why OWC offers optional port tighteners with its USB-C docks. I wonder how USB-C connectors fare in professional settings, where they could be subject to much more abuse than I inflicted during my careful connection and disconnection for drive testing. The females should really be built with thicker outer metal walls. As you can't make the shield walls of the males much thicker, they need to look into stronger metals.
3) I replaced the San Disk Extreme with a pair of Kingston XS2000's. They are slower than the San Disk, but don't spontaneously disconnect when attached to the TB3 ports. The SanDisk was faster than the XS2000, but draws more power (25% more for the same drive size, according to Anadtech's measurements), so that could be why. Indeed, the XS2000's are suffiicently low in power that, up until a few weeks ago, I was able to run them from my USB-A ports, freeing the two TB3 ports for my two external monitors. Alas, that all changed when I upgraded from Monterey 12.7 to 12.7.1. Then both drives began repeatedly disconnecting. And not only that, but I began to experience global OS problems, including routine cursor freezes (every few hours, forcing me to reboot), and a couple of kernel panics [
panic(cpu 0 caller 0xffffff80037876a2): nvme: "3rd party NVMe controller. Command timeout.]
Switching the two Kingston XS2000's from the USB-A ports to the TB3 ports fixed all those issues. If power delivery were the issue, that could explain why—TB3's 15 W limit is >3x the 4.5 W limit of USB-A, which are USB Gen 3, and is well under the 4.73 W max
Anandtech measured in their review of the 2 TB XS2000.
It is mysterious that plugging bus-powered SSD's into USB-A ports should create global OS issues, as well as a panic involving my internal NVMe drive, as those would seem to have nothing to do with the USB ports. But one reason drives could disconnect is because they are drawing more power than then the ports can (or are allowed to) supply. So I suppose it is at least possible that the Monterey update affected that power setting, and it is further plausible that something that causes a port power limit to be exceeded (thus forcing the disconnects) could have global effects on the PCIe system (which feeds the internal drive as well) (but of course I am purely speculating here).
Anyways, since I now have four devices that need to be plugged into TB ports (my two Kingston XS2000's, and my two external monitors), and my iMac has only two, I got a Sonnet Echo 11 TB4 dock (on sale during BF for $160). Its three downstream TB ports gives me two more TB ports (I lose one to plug in the Echo), for a total of four.
