Also it's a great test for the EU. If all the EU countries can still actually agree on anything so novel. If we couldn't all agree on obvious stuff like support for Ukraine (famously Hungary had issues).
Even if things are very positive on the surface there are often small groups that get affected negatively. We can see this on the recent example of the EU-Mercosur agreement. It was supposed to to "unite a free market of 700mln people", but for example farmers in my country (Poland) protested it an I think they got a lot of concessions. (We have to remain food-self sufficient in case of war, so I'm not against their ideas per se). The deal is supposedly done, but I'm still not seeing bananas from South America in our supermarkets (don't laugh, this was one of the main points anti EU people raised back in 2005).
Ideally I'd like to see free market eventually expanded to free movement of people between EU/Canada but I have no idea how difficult aligning the legal systems might be.
As to contributions, it's not membership so things can maybe be worked out. As to the UK being able to get similar deal. I don't really get the idea to "punish the UK for exiting" they shot themselves in the foot sufficiently anyway (as well as us), but Canada has way more natural resources which may be a great benefit to our common market. So they have better "cards" to quote a certain Donald.
For example the whole farce that is EUs green initiative could actually make sense if we had a place to procure resources from in a more ecologically sound way. As opposed to what we do with China, we basically killed 90% of local industry, but we still get finished goods from countries that burn huge amounts of coal and manufacture those things in ways that would be never allowed here.
Of course that we E immensely expensive to set up, but we have 2 trillion $ in investment looking for a new home and there is a good argument to make this should be invested in a place that honours rule of law, democracy, human rights (inclusive of right to healthcare) and so on.