As for the bus thing, OtE, I find buses very stressful to say the least.
That being said, on my local buses, the unwritten rule most people seem to obey is that unmasked people go upstairs or at the back, unless they need to be in the buggy bays.
I don't know what the situation is in Wales, but here we've got the rules that cover most of England (but not, I believe, London?) whereby there are no laws or bylaws requiring masks on buses, but bus companies are free to set their own terms and conditions.
Various bus companies operate near me, but all require masks. But as with your local buses, drivers aren't making a fuss with unmasked passengers.
I can't see what they're supposed to do, though. As you point out, if people find they're getting challenged, they can always go and buy a sunflower lanyard. It might have been to the point to get a system in place for applying for and getting some sort of exempt status, but the government wasn't going to be able to do that, even if it wanted to, having already effectively destroyed our health assessment system for other purposes such as benefits.
Also, whilst I find it frustrating when people don't wear masks, in all seriousness, when you read what's written online, I do believe that a lot of people out there now believe that wearing a mask is dangerous because it makes you re-breathe in your own exhaled breath and...well, the explanations then vary. So if you've read or heard over and over and over again that masks are dangerous and harm you, why mightn't you become genuinely afraid of wearing one?
How would a formal exemption scheme handle that?
So whilst I want to scream my head off not just with fury but with, genuinely, fear when I get on a bus with lots of unmasked people, I think communication's been handled in a way that means that if now we had a formal assessment for exemption scheme, lots of people would be genuinely exempt now who wouldn't otherwise be, because they are genuinely scared.
That doesn't fix your bus problem, just maybe next time you get on a bus, maybe it might help a little to know you're not the only one feeling frustrated.
Oh, and the lanyard thing - different aspect - abuse of it is particularly problematic for those of us with hidden disabilities, isn't it?