Well at some point there are no viable software-related fixes to try. It's not a common occurrence but hardware failure issues do happen so in this matter if the Bluetooth radio chip has failed, there is no software solution, it's hardware. Since you're only providing details and things you've already tried on a limited basis I'm out of ideas (or at least time to post what are just multiple, repetitive suggestions).
Typically with a hardware related failure, that involves a full to partial tear-down of the phone. And it's not something a novice should try as an experiment. I'm not familiar with a Nokia G21 and since it's very manufacturer and model specific, this could be a big issue if you're your paying out-of-pocket. If you're lucky, the Bluetooth chip is on an easier to replace, separate daughter-board or something like a network related, accessible module (WiFi, cellular, and Bluetooth radios in their own board). Otherwise it might involve a complete replacement of the entire logic board. But this is if the matter is actually hardware related, with remains to be determined. Hopefully others will suggest a fix that does work. But if you don't have the time to drop off your phone to have it looked into, your options are to keep trying to solve this matter using software-related solutions, or accept the fact that Bluetooth audio isn't fully functional on this particular phone.