Well I have to confess I'm also in my mid-fifties and have noticed I've started having a harder time learning new things, have a harder time taking in conversation, etc. Apparently this is a thing that happens as we age. However, it's documented that hearing loss has negative effects on cognition, and that hearing aids help slow that cognitive decline. I think it's a reasonable extrapolation that, just like muscle loss, the more you slow the decline, the better you'll do in the long run. So I would push the hearing aids. If you are going to threaten to split up, that might be the place to use that leverage because it will likely benefit him in a material way.
It's also known that people who start with high intelligence and high capabilities in other intellectual areas, and who have mentally interesting jobs, have slower mental decline and are functional (mentally) for longer. I believe the term for this is "cognitive reserve."
So if you have always been the one with the job that required more mental capacity and his job has been manual, then this current situation is probably somewhat expected.
However, that does not mean you should have to jump to his tune and be exhausted all the time.
I would also say that leaving the hob on and almost burning down the house by setting a tea towel alight, is not something that you can live with. You are not his mother so I would ask him what he plans to do to ensure his behavior is not unsafe in this area.
Him always getting his way, and you having to run around getting exhausted trying to do everything yourself, is not workable. So I think you can state that you cannot continue like this and he needs to be willing to change some things.