Okay making some progress on understanding my lack of 802.11ac in FreeBSD 14.3. "Computer says no".
$ sudo ifconfig wlan0 mode 11ac
ifconfig: SIOCSIFMEDIA (media): Device not configured
... whereas forcing 802.11a "just works".
$ sudo ifconfig wlan0 mode 11a
I think I've somehow fouled up the upgrade from 14.2.
( Ongoing discussion here: https://forums.freebsd.org/threads/slower-than-expected-performance-from-intel-wifi-drivers.98118/ )