Bluetooth headphones suddenly too quiet

I have bluetooth headphones with a noise cancelling microphone which I need for making phone calls through my Mac in a co-working space. The headphones are high quality Sennheiser MP Pro 1, work fine on my iPhone, and for years worked fine on my Mac.

A year or two ago the max volume when used on my Mac (but not on my iPhone) massively decreased, and they are now unusable. I'm looking at buying a replacement, but that seems ridiculous as they are expensive and what's to stop the replacement doing the same thing. I'm pretty sure the problem is a Mac issue - a few other people have posted but there are no solutions that I can find.

I have removed them through "forget this device" and reconnected them - but that hasn't solved the problem. I hoped it might go away over time if a fix was rolled out, but it hasn't.

I would just use AirPods (which still have normal volume), but the microphone is not sufficient as it picks up all the background noise in the room.

Does anyone have any advice please?