I mostly want the M5 Pro because my choice of an M4 Air this year with 24 GB of RAM is turning out to be less than I want with the things I'm doing these days.

The former is already true and will become even more true when M5 Pro/Max/Ultra release.

RDMA is a networking standard, it's supposed to be switched. The reason why it's being done over Thunderbolt is that it's the only cheap/prosumer I/O standard with enough bandwidth to make this work. Like, 100Gbit Ethernet cards are several hundred dollars minimum, for two ports, and you have to deal with SFP+ cabling. Thunderbolt is just way nicer[0].

The way this capability is exposed in the OS is that the computers negotiate an Ethernet bridge on top of the TB link. I suspect they're actually exposing PCIe Ethernet NICs to each other, but I'm not sure. But either way, a "Thunderbolt router" would just be a computer with a shitton of USB-C ports (in the same way that an "Ethernet router" is just a computer with a shitton of Ethernet ports). I suspect the biggest hurdle would actually just be sourcing an SoC with a lot of switching fabric but not a lot of compute. Like, you'd need Threadripper levels of connectivity but with like, one or two actual CPU cores.

[0] Like, last time I had to swap work laptops, I just plugged a TB cable between them and did an `rsync`.