They could set up an account on one of the larger well established Canadian instances or even better start up their own.

Both of these options have their pros and cons, and I think it is important to explain these well to the council if you want to have any hope of convincing them.

A line of argument that has had some success in Europe is what has become known as "Digital Sovereignty", basically a fancy term for saying government should control its own infrastructure. So you might want to sell it as an easy way to have a permanent archive of public communication and a method for it that is under their direct control, rather than as a way to find more engagement.

As others have said self hosting has a maintenance and moderation overhead, but this can be lessened by running an instance together with other cities while still retaining most of the benefits of self hosting.

Seeing from the linked cross-post that this is about Port Alberni, and considering that http://portalberni.ca/ returns an empty reply while https://portalberni.ca/ lets me know I have been geoblocked because I'm outside of Canada and the US, I'd say you have an uphill battle before you though. These people made a website (probably paid for it, too), and then killed much of its use by geoblocking most of the world.

Good luck.