I've literally just taken a trip to Canada from Europe at 32 weeks with my second, so it's fresh in my mind.
After first pregnancy I would have said "no worries" as I had an easy time at this stage. This time, not so much.
I did get business class (never had it before), and have to say with the benefits, it was still uncomfortable- dread to think how I would've been even in premium economy.
It's not just leg room, it's the ability to recline the seat enough. By 30 weeks plus, I've found it very uncomfortable to stay "sitting" in terms of stomach/bump for very long, even if you have some space to move legs. You will probably have to get up and down all the time for comfort as well as health, and so an aisle seat would be a must. Also, you'll need to bring a lot of extra drinks on board to combat dehydration (or budget to buys lots from cabin crew), which is important for DVT risk too. Plus of course you should wear proper fitted compression stockings (uncomfortable things though they are).
Also, consider that just walking round the airport with hand luggage may be a big deal by then; I found it pretty exhausting and even painful (from joints, minor SPD) getting between different terminals, and was glad my layover was enough time to walk slowly, and I could access the business lounge to rest properly. Think about if your flight's delayed and you have to rush... not fun and potentially dangerous.
Also you MUST have fully comprehensive insurance- Canada has high health costs, and you need insurance that will cover the possibility of you actually having the baby over there, and both needing special care for a long time etc, plus flights back home afterwards.
I would say if there is no chance at all of business class, then you should only provisionally agree, with the understanding from everyone now that you can back out at the last minute if you feel at all like it will be a bit too much. Not only did I find the flight hard, but 5 days running workshop whilst there (plus jetlag...) was also totally exhausting. Don't get in a situation where you feel any pressure to go in several months time, it's not worth it.