@brooke Yeah, I found that RTL8156BG-based USB 2.5gbps adapters Just Work, and get full speed. I've got the USB-A version of the UGREEN adapter on Amazon, but they have a USB-C one too.
(Accidentally found it, even. I bought it a few years ago because it was cheap gigabit and used it in my router, only to find it was actually 2.5gbps when I got a 2.5gbps switch a few weeks ago.)
@foo @brooke just make sure to get the RTL8156BG and maybe not the B or the original; the older chips apparently run hotter and have more hardware bugs, while the -BG may be less well supported on Linux kernel versions pre ~5.4ish
source: just bought a second of these and will soon buy a third. other semiconductor manufacturers have released USB to 2.5 GbE chips, but it seems like the 8156 is the only one that wound up in actual dongles (according to my limited research)