I think I found a solution to this issue, but I'm interested in feedback on it.
I modified /etc/nginx/conf.d/gl.conf and changed the ipv4 listen line from:
listen :443 ssl;
to
listen 192.168.8.1:443 ssl;
(Replacing the 192.x.x.x address with my router's own internal address)
It seems that now I don't get the CORS errors, and I can still access both the gl.inet and luci interfaces. Since I only want the web UI accessible from inside my network (and via VPN, which also works), this change makes sense to me.
But I'm not sure why it fixed the CORS errors. I made the change while trying to resolve the other issue I asked about on Friday (Wireguard connection to web services in my LAN using external IP/DNS address) and found that this change affected this particular issue.