Thanks for your reply.
Good news everyone - I got it sorted.
The issue is that the DJI GO 4 app version 4.3.46 (android 12) doesn't allow this hack anymore. By the way, version 4.3.46 is only available from the DJI website. The previous version 4.3.37 is downloadable from google play store - but alas it does not launch on Android 12 (Pixel 5). It installs, but won't launch.
Luckily, I still had an older phone on Android 11 (pixel 2) in a box somewhere. This phone still had DJI GO 4 version 4.3.37 installed on it. I installed the fake GPS app on the old phone (no sim in the phone, but I could connect to google play from my wifi). I followed the standard hack procedure as outlined above. It worked. I chose OK to change my WLAN settings to US region.
Now the real test. When I plug the remote back into my Pixel 5 (Android 12) with version 4.3.46 on it. Would it still remain in FCC mode.... YES! The pop up appeared '
your aircraft is flying in a different region...'. I chose 'cancel' when prompted to change my WLAN settings. I confirmed the wifi channel screen still shows only 11 2.4ghz channels (FCC) not 13 (CE).
Now the real REAL test. I took a flight on the very same route I did earlier today. This time... No grey screen. No aircraft disconnect. No weak signal. No issues at all out to 500m (as far as I went). Not surprising given the FCC power output is more than double in 5.8ghz mode and almost double in 2.4ghz mode. (see stats highlighted in Mavic air spec sheet).
I hope this is helpful for people. You will basically need Android 11 and DJI GO 4 version 4.3.37 to apply the hack. (4.3.37 won't launch on Android 12!)
The hack cannot be applied using Android 12 and DJI GO 4 version 4.3.46 (only version that launches on Android 12).
However, once the hack is applied you can use 4.3.46 on Android 12 - just be careful to always hit 'cancel' when asked if you want to update your WLAN settings!