"A bit of background, she has been my line manager for 18 months since a restructure. I foolishly didn't go for the post for personal reasons, but have in effect been doing her job all along. She has had long periods of sickness, but was able to complete an album for her band during her last one as well as perform on stage at gigs. She is not interested or capable of doing the job. Fair enough she is sick, fair enough she in not interested. I know I should leave but sadly for me it is a job I have loved for 6 years and I have a great team of other colleagues.
I have always shown her respect and support until the past 2 months when I have realised she is taking the piss. In 18 months we had two 1:2:1 meetings and three team meetings, so she has shown me no respect as a line manager."
"Maybe I should talk to HR on Monday. Maybe speak to my union too but I don't think they would back me. 33 years in the union and they will shaft me too.
I feel really fed up with this, I have been so stressed over past month with this situation- before she even declared her specialness- not sleeping, worrying. DH and DC v supportive but getting fed up with topic as I go over and over it at home. And now this is the final insult 😥 "
"I'm the only one she manages. It's a small team one other member who is line managed by someone else.
I have raised my concerns with "management" don't want to say to much on here..."
The "Pronouns Thing" is on top of all that prior piss-taking??
"In 18 months we had two 1:2:1 meetings and three team meetings, so she has shown me no respect as a line manager."
So she is failing to fulfil her contractual obligations and you have raised this with management but they have not (to the best of your knowledge) taken any action.
"I feel really fed up with this, I have been so stressed over past month with this situation- before she even declared her specialness- not sleeping, worrying. DH and DC v supportive but getting fed up with topic as I go over and over it at home. And now this is the final insult 😥 "
Your health and your family are more important than this woman's pronoun nonsense. She is being paid to manage and she is not doing the basics of 1:1 supervision and team meetings.
It is easy for me to say, "I would . . . " but . . . in the circumstances, I would seriously consider a visit to the GP with a view to getting signed off for Work-Related Stress.
If your work has an Occupation Health Dept. they might want to see you, which will give you the chance to pour your heart out about the grief she has given you. Seriously, "pronouns" sound like they are the last straw rather than the main issue and therefore not the thing to focus on if you take this course of action.
HR might be more inclined to step in and do something about this situation if it is brought home to them that the organisation has been failing in its duty of care to you. If you can have some time away from work to recover then you should also be better able to deal with the situation.
For example, if the woman has been failing to provide adequate support and supervision and you have already brought this to Management's attention then it would not be unreasonable to follow up by asking what they are going to do about it. HR or Occ Health might do that for you. Even you GP might write a supportive letter. If there is someone else who could provide support and supervision then that might get her out of your hair altogether.
By the time you get back she might be gone anyway. Neither Management nor HR are going to be happy about a good worker driven to illness. Apart from lost productivity they are automatically going to be thinking about the possibility that you might bring a Grievance.
This is and should be about you, your health, your workplace - not her. 
ps. NEVER resign without getting union advice first. As you are a union member it would be worth speaking to your workplace rep/shop steward and dropping them a line so you have something on record. Personally, I would not mention the "pronouns thing" as this has arisen after you have already been made ill with worry by the situation.