The fix you had originally, create the config file which disables hardware volume control for that headset,
is the correct one. Kinda.
There is a typo in that quote - spot the missing ‘.’ (confd should be conf.d)
If it’s not that, it is likely that it is not working because you are using kinoite and it does not use the config files from the standard location (common among immutable distros, I’m unsure if it applies there). If you ask about this on a kinoite forum they will tell you where you can put the config file.
If you use other BT headsets, you will want to improve on that fix. The config you have there, disables hardware volume control on all of your bluetooth headsets. Hardware volume control is nice to have with wireless devices because it saves battery.
Sometimes, there will be a bug (you should report it, it might be fixable!) or the device may simply not support it, and the headset might need hardware volume control disabled, like yours maybe does… but generally, it is better to keep it enabled. You can create a rule which applies only to that device, and disable hardware volume control specifically for that one (or only for certain profiles, depending on what your device needs).
And the best part - this should all be handled automatically and is done so based on a hardware database maintained by the wireplumber team. If your headset needs special treatment, if they have your headset on their list, it’s taken care of by magic. If you file this as a bug they can fix it so it just works.
Hope that helps.
Edit: LOL and I just realised that I was the guy in the other link 