Thank you very much, John W (and to Ric, who had tried to steer me to the same facts earlier). You've answered my question fully, by pointing out there's no need for me do the search for such an enclosure or drive in the first place.A perfectly reasonable question, but to pick nits, it's probably worth noting that Thunderbolt 3 is essentially a PCIe 3 x4 connection, and this isn't sufficient to support "the fastest SSD drives available".
It's fast enough to support virtually any existing single M.2 NVME SSD blade at full capacity, because these devices are also based on a PCIe 3 x4 connection. But anything faster - a high performance PCIe datacenter SSD, or a striped (RAID 0) array of multiple M.2 blades on a card - will be performance-limited by Thunderbolt. These types of things would need to be plugged directly into a sufficiently fast/wide PCIe slot to hit full speed (and so aren't really worth the cost if your plan is to put them in a Thunderbolt 3 enclosure..)
When the time comes, I'll simply convert my existing Thunderbolt 3-enabled OWC ThunderBlade to JBOD, which will boot Catalina.