When to seek for professional help?

Absolutely. Why not.

If you are unhappy, seeing a therapist seems like a perfectly reasonable thing to do. You can't force your husband if he is not on-board. You can take care of yourself.

Seems like you’re putting the cart before the horse, borrowing trouble etc when you haven’t even talked about this with him.

That seems like the logical first step, rather then wasting time wondering “what if?”

Presumably you've tried everything you can think of to work things out / get them headed in the direction that will make you happy.

If he doesn't want to attend counselling, then it's a good thing for you to attend by yourself - in order to help clarify things for you. You are after all, seeking counselling / clarity for yourself here, on a forum. No different in reasoning to seeing a counsellor, except they are trained, can dedicate time to you, can engage in quick question & answer exchange, etc.

Do you think your husband would not “be on board” to counseling?

You say you are not sure how he feels about the marriage. Has he expressed unhappiness about what’s going on?