Visiting relatives in Canada for 2-3 months, need I be worried about the stuff I bring along?

I wouldn't worry about being charged duty. I would worry about being turned away because the officer believes (correctly) you would look for work. People generally bring that kind of equipment so they can work.

My advice is to decide whether you want to visit or work. If you want to work, then do the paperwork now. You can't get a work permit when you are here or switch to working after being admitted to visit. If you don't want to or can't get permission to work, then decide you won't work. Bring what you need to do a family video and leave the rest behind.

There's nothing wrong with visiting a place to see if you want to move there. But you would need to return home and then go through the process to move. You can't turn a visit into a move. Leaving your filmmaking equipment at home is one way to demonstrate that you're not moving, just visiting.

This could be considered the equivalent of a tradesman trying to enter the country with a toolbox and work clothes. It looks very much like someone who is intending to work illegally and could result in being turned back. Legitimate short-term visitors do not generally have such problems.

Suggest you bring a reasonable amount of non-professional stuff and if you need some during your visit consider renting it. That said, a DSLR camera a whack of lenses and a Macbook is not that much stuff, compared to say a professional video camera and lighting gear. What you say upon inspection and how you back it up with documentation etc. will be more important.

As far as duty goes, I don't think there generally is any duty on photographic equipment made in Japan, but there would typically be sales tax of 13% or so if you were to be permanently importing the goods. If it was deemed to be for sale, a commercial entry would be required, which might not be all that straightforward for you. But your main worry I think is immigration status not customs duty.