Wow, Albireo13, I did not realize we are about the same age. Another post made me think you were much younger. Now I'm reading through your previous posts with interest. I'd like to finish before commenting much more than this. If you're interested in what I hope will be helpful feedback, please don't lock the thread just yet.
What I see so far is that you have a lot of stressors in your life. It isn't just the job that's "turning your brain into mush". You seem to be facing a wall of stress from all directions.
I would definitely turn down the pseudo-promotion. You just got a raise, you don't need the money. It's fine to indicate to your boss that you want to enter the glide path toward retirement. You might also cite the challenges of dealing with aging parents.
Perhaps you've written about this elsewhere, but I haven't seen it, so sorry if this is a repeat. Could you explain a little bit more about your youngest son?
My bonus kid is about to turn 26, still in Junior College, and only started a real-ish job about a year ago. It's not full time and doesn't have benefits. And he still lives at home. DH also lost his first wife and received a similar amount from her 401k. We have also dealt with the aging process and death of three out of four parents in the last five years. The remaining parent has ALZ and lives with us. Seems we have a lot in common... Oh, and I hated my job before retirement ;-)