Okay, this is a stupid one because I was looking in the wrong direction…
# When using the embedded outpost, use:
proxy_pass http://authentik.company:9000/outpost.goauthentik.io;
This ended up being me looking in the wrong direction. It should have been as follows:
# When using the embedded outpost, use:
proxy_pass http://192.168.1.2:30140/outpost.goauthentik.io;
This is defined as the http port internally for Authentik in TrueNAS Apps. Whoops.