Answering my question with a no extra cost solution using the Noise cancellation headset I had.

Using any Bluetooth headset (even supporting BT 5.0) like Crossbeats and recording with stock camera app or with Open Camera with audio source set to external doesn't work.

Solution

• I had a Bose BT2 Bluetooth headset, which I haven't used in the last 7 years and surprisingly charged up and gives me about an hour of usage time which is more than enough. I wanted to use this to improve the quality of recording since it has excellent noise cancellation. You may as well pick up any recent BT headsets of your choice and of recent vintage. Profiles supported by Bose headset are HSP, HFP, A2DP - SNK, AVRCP

• Download Bluetooth Mono Router. Blogs and YouTube videos recommend BTmono but it hasn't been updated for Android 10 and won't work. Both these apps have the same functionality - they let you play BT audio to those old headsets which only support voice calls and not media. I am not clear as to how this let's you overcome the limitations of recording but it works (someone here may have an explanation) . You can use either stock camera app or Open Camera (audio as external in settings)

• In case you can't record, activate developer options and from Bluetooth settings try AVRCP 1.3 and SBC codec combination. These seem promising. In my case I used default

Permit me a little rant about One Plus tech support with interactions over several mails. YMMV. Documenting it just in case it helps others.

• They do not respond when you ask them how to record using Bluetooth.

• Using wired Lavalier solutions need an extra adapter in some cases and even then no guarantees. Reason : Non standard implementation of USB C to audio adapter (Shades of wrong resistance used in early models) . They claim on mail that TRRS is not supported since it is a third party affair and when confronted with standards do not reply