it's not that radical, just kinda expensive up front, but you can do it in sections. connect toronto to winnipeg, buy the land, start your seed cities use the funds to do the winnipeg to calgary line (optional later extending to edmonton) and then to vancouver repeating the process. the HSR makes them functionally suburbs of the bigger pre-existing cities they connect to, and with housing prices like canada that's immediately attractive on it's own. make sure to cooperate with the USA and mexico to establish a continental standard for compatibility for HSR to prevent future issues.
seed jobs, enable population-based service jobs like teachers and restaurants move in to capture demand and provide further jobs. now you have an economy in these cities and people will start moving there for non suburb-reasons. offer tax breaks to companies to move to these new cities, and raise taxes generally on businesses to make it revenue neutral.
build 20,000 public housing units in each city to provide immediate housing and establish a starter housing stock, protect against homelessness and kickstart the construction industry there (also jobs) and then the companies you paid to do that can go on to build privately owned housing
optionally but highly recommended is to build walkable, bikable urbanist cites that are what the younger generation (older people are mush less likely to want to uproot, espically if they already own a home, so they shouldn't be the primary target audiance) is looking for, and also do green construction that fits in well with the climate of each location. these aren't required per se but you may as well. canada has the same auto dependency problems as the USA and this is a great chance to provide an alternative
you can do this in the USA too but canada has even more immigrants per capita so if someone's already moving to canada from abroad they can just choose to settle in one of the new towns for the same effort as toronto. honestly we need a fuck ton more housing in general, not just for the native born population to have a snowballs chance in hell at moving out of their parent's place and starting a family, but also there's gonna be a big global refugee crisis because of climate change being ignored for so long so those people gotta go somewhere