Poor girl.
Thank you for the video and photos.
I would try to make her as comfortable as possible, then let her go when she declines further.
I would suspect something reproductive going on with her. Sadly, these things are common in laying hens.
If you lose her and want to find out more, a necropsy through your state lab will give you the most information or you can do an informal investigation like some of us do, looking at the internals to see if we find anything obvious.
With her age, 8 years is quite respectable and not all hens make it to that age. So, you have done well taking care of her, she's had a long happy life. I can tell you care deeply for her and it's hard to lose them, you do become attached for sure.
If you do decide to put her down, hopefully you can find a vet willing to help, if not, let us know and we can point you to a few methods. I put my own girls (and boys) down, they know me and I care for them, I can't stand the thought of a stranger touching them in their last moments.