How to discuss with boss about not being happy at work anymore?

[OlgaDePolga]

So.... bit of a long story but will try to be succinct:
New line manager started about a year ago, they have taken away any autonomy I once had and question every decision I make to the point of arguing/talking over me in team meetings. This is on top of them making rude comments to other colleagues about my mental health and trying to gaslight me into questioning my own actions.
Anyway, my old line manager and managing director have now asked to have a meeting with me to 'have a catch up' and I want to raise this issue but not sure whether to raise it as a bullying issue or as a bad management issue iyswim.
Been the longest standing member of the team and had the most experience in my field. It feels like this line manager has turned all that experience into nothing as any feedback I can give based on previous experiences is rubbished.
Am currently looking for a new job - should I tell the bosses and see if they'll try to help the situation or just give up and high tail it out as fast as poss?
Small company which the line manager is in real danger of ruining thanks to his breeding a toxic culture of bullying and harassment.
TIA

Start off with feedback in good faith and give him a chance to repair things. Once he's been given feedback if he still doesn't improve then you may need to look at your company's grievance process.

Golden rules of feedback:

Keep it factual and specific. Don't infer, think very carefully about what you know, and what you may have assumed. Getting overly emotive or basing things too much on feelings can make it easier for people to dismiss what you're saying. A good mnemonic to keep in mind is SAID:

SITUATION - outline the situation. What happened or is happening. Keep this to things you have observed, seen, heard.

ACTION - what are the actions involved? What is the person doing that you feel is relevant? Again, keep this to actions, not personality traits you have inferred from those actions.

IMPACT - what is the impact of those actions. Here, you need to keep to 'I' statements. Avoid any accusations that might make it look like you're assuming or guessing and keep to 'I feel' or 'I find'.

DO - what is it that you would like the person to do? Focus on tangible things they could do to improve the situation.

Thanks so much for a quick reply, that's really helpful and I appreciate you taking the time to post

It is possible that the catch up is because they have noticed your new line manager is doing a poor job - Giblet has given you very good advice.
As a small business owner - we would want to know - actually we would already know, we'd just be considering our options regarding how to fix things and having a chat with you would be part of that. If they aren't interested in fixing things be glad you found out now who they are and find a new job where you will be valued.

Thanks for your reply @TarnishedSilver - it could be that... I have tried raising various issues that have cropped up before with both these people (old line manager and MD) and they haven't really listened. I am really regarding this 'catch up' as one last chance to resolve the problems before I jump ship. I think it's the lack of value I seem to have that hurts the most... been with the company a long time, watched it grow from 9 staff to 30+ and worked my nads off only to find myself pushed sideways all the time, no real room for career advancement