my line manager has been effectively bullying me since 2020
let's take this to its extreme @HR313 - supposing you took this statement into your grievance, you described all the occasions since 2020 where your line manager has bullied you, made you feel threatened, vulnerable, demeaned etc (whatever she did).
You need to consider what outcome you need from submitting the grievance.
Unless you can give some cast-iron case of discrimination, for example that she treated you less favourably than male colleagues, there is no case for Tribunal. So what then? She will never capitulate and apologise or admit her wrong doing. You can only rely on your company's internal policy around bullying. In my experience these are very woolly and ineffectual with no repercussions on the bully other than a behind-closed-doors conversation (at worst).
And if it were to go to Tribunal, they would argue "why did you put up with this for 4 years?"
so your best option is find a new job, try to leave with the ability to get a bland reference from them if your new employer wants one, and look forward to a better future from this toxic manager.