It's ok to be unsure about what you want. Six years is a long time.
You talked in your OP about a 3month separation where he gets help for his problems and doesn't contact you. Are you still thinking about that ?
You say he's still trying to blame you, which doesn't fit well with your hope of him accepting responsibility for his issues and getting help. If he's done nothing wrong and it's all your fault , why would he get counselling?
BTW You don't have to be perfect in all this, no one is perfect. You just have to be a decent human being, which you know you are.
Regarding your "heavy conversation " - you can do this in whatever way is best for you .
If you want a divorce, I suggest you stall for now and get legal advice first. You know that he's manipulative and dishonest so he's not going to be fair. You need to get all your ducks in a row first.
If you just want a longer separation with conditions, why don't you just email him now? isnt it better to have it in writing for clarity and he's less likely to try to manipulate you ?
its not like it's going to come as a shock to him . He's been cheating for years so he must have known that you would find out sooner or later.
You know him best. Is he likely to become violent or abusive ? If so, don't do it in person .
If he likely to shout or argue with you or blame you ? If so, why do you want to have such a conversation and not just phone or email ? He doesn't have some moral right to have a show down with you.
Are you thinking of having a counsellor there to suport you or him ? Is this a counsellor you are already seeing or is it HIS counsellor ? Are you going to pay for this?
Or are you thinking that the counsellor will act as some sort of referee and allow you a chance to say your piece ?
I would have thought that a manipulative man like your H would try to get the counsellor to feel sorry for him - how would that make you feel ?
You could also have a friend or family member there instead, but He might act up in front of them as well.
I guess I'm wondering why you want to have any kind of conversation at all with an angry , manipulative , cheating , dishonest man who blames you for his infidelity?
Do you want to put yourself through that ? What's the point?