Apparently macOS doesn't expose the ability for userspace to manually issue trim commands to block devices (or at least doesn't ship a suitable tool to do so), so the best available workaround is to tell the filesystem layer that it should do automatic trimming even on external drives.
https://kb.plugable.com/data-storage/trim-an-ssd-in-macos
Apparently macOS doesn't expose the ability for userspace to manually issue trim commands to block devices (or at least doesn't ship a suitable tool to do so), so the best available workaround is to tell the filesystem layer that it should do automatic trimming even on external drives.