It's been several months, and many, many operating systems (BSD's, linuxen, illumos) on my spare T420. I decided to put NetBSD 9.3 on it recently.
Wifi is working! I just wish I had an emoji of me banging my head against a wall for not figuring this out at the time ;)
What I did to make it work. Firstly, I stopped trying to connect to my local 5ghz network, and just use the 2.4 network. I enabled wpa_supplicant in run time, and then dhcpcd as well. When I was sure everything worked I added the relevant info to rc.conf. On reboot everything worked as expected.
A tip for anyone who might come across this thread: Slow down and be patient. I have a lack of patience at times, which definitely stopped me from resolving what was a very simple problem. Hence, why I don't work in IT.................
As of now everything works. Relevant rc.conf for anyone who might see this in the future: