Pre-industrial ethnic groups barring a few special cases mostly get hemmed in by geography alongside that sort-of limit of solidarity you mentioned. Ireland is an easy example as a small island, but the biggest unified (so to speak) nations/peoples in common human history outside some special cases are say European nations like France or the extent of pre-WW2 ethnic Germany (+Austria and Luxembourg) and Poland, or the sub-nations and ethnic groups of India pre-British Empire. Even if regional differences inevitably occurred, they still thought of themselves as more or less one nation and country - witness how politically the Holy Roman Empire was frequently called simply "Germany" on maps from the 1500-1800s. Pretty simple so far, right?
Special cases involve what could be called eternal or "civilizational"-states, big enough to be subcontinental in their scope: China Proper (where 99% of Han Chinese then and now live and Chinese dynasties always controlled since Imperial China formed), Iran/Persia (the Iranian Plateau and till recent history also the "Stans"), and the Roman Empire's core borders come its own Imperial period. They too involved easy-to-cross internal geography (Mediterranean Sea, North China Plain, Iranian Plateau's raised nature) and protective external geography (Rhine/Danube/Euphrates and Sahara Desert, Himalayas/Gobi Desert/China Seas, Zagros/Elbruz Mountains) that allowed them to conquer all the rival tribes in the internal part of their geography and gestate a singular nation/people, if with those inevitable regional differences, on a simply much bigger scale...alongside that singular sense of self meaning if split-ups (inevitably, of course!) happened - they WANTED to reunify as fast as possible (if preferably on their region's terms)! And that bigger scale allowed them to frequently absorb and assimilate inevitable invasions from nomadic hordes or would-be conquerers (China's famous for it, but notice Iran simply moved past the Selucid Greeks and kept a cultural cohesion outside Islamic conquest even forming its own sect of it eventually, Eastern Rome was on the way to reconquering the lost west when the Islamic invasions took its attention and so let the west coalesce into truly separate nations). Also note these particular special cases/super-sized states forced standardized forms everywhere: a strong central government over provinces that had some local flexibility, a standardized language in spite of inevitable dialects, a common religion or sub-sect of one, etc. to get past those regionalisms. But that takes time to do even internally. Rome WAS well on its way (look at Romance languages and Catholicism, the Greeks clinging to the "Rhomanoi" name, and the western obsession with "universal empire") but unluckily collapsed at the cusp of that gestation truly taking hold, while China Proper and Iran got past their split-ups in the same time frame to successfully become one land and people, seemingly forever.
The USA's possibly-probably a modernized version of this if you magically made it form in pre-industrial (well, it more or less technically did, see below) or even pre-gunpowder times: east of the Great Plains alone is a China Proper/Roman Empire sized land area already with 70-75% of the population, the Mississippi watershed and all the rivers and canals therein with the Great Lakes/Atlantic Ocean/Gulf of Mexico allowing easy sailing everywhere in the area even outside easy-to-cross-on-foot/horse arable land, and in spite of modern Democrat/leftist subversions America famously forced assimilation when possible on a singular "White" identity with Anglo roots (re: speak English, practice Protestantism) in spite of massive Germanic (German, Nordic) then Catholic immigration (Irish, southern German, Italian) immigration somewhat later into its history, and finally the Revolutionary and Civil Wars gave a powerful impetus to seeing itself as a singular nation. And even if you extend this to gunpowder times as it actually was in reality (which makes steppe invasions a thing of the past) that's easy-to-cross plains and then the South Pass into the west and Pacific Northwest/Columbia River-to-the-Pacific in particular that makes the Rockies suddenly neligible to cross. In the course of the Revolutionary generation's timeline (1750s-1820s), the last one before serious industrialization hit North America, they went from merely living on the Eastern seaboard to settling westward the Plains/Missouri/Sabine/Red River/Ozarks with strong claims on Texas and Oregon partly based on that rather-easy-to-cross geography. And externally, of course, you have the oceans, the Great Lakes and Appalachians, the Chihuahuan and Wild Horse Deserts with the Rio Grande/Gila Rivers, etc. etc. depending on technology level we'd have the USA dealing with here.
Canada? Despite mostly being an "Anglo' country, its main geographic lifeblood is the St. Lawrence and Great Lakes.... inhabited by Quebec, which is separated from the east by the Appalachians anyways. The Maritimes's first Anglos were New England Planters and Newfoundland's always its own thing, Ontario is de-facto a peninsula clinging to the Great Lakes, the Rockies of Canada have no South Pass like the USA does so British Columbia's its own thing, the Plains as you noted had serious Uke settlement (and much more proportionally important than the famous German/Nordic settlers of the American Midwest: there were still tons if not a genuine majority of Old-Stock Americans settling there!). No Mississippi watershed equivalent for easy sailing outside the Great Lakes that don't even have a useful canal or river to Winnipeg, so it's more distance than you think traveling by land to the Prairies even if it's not difficult per se. Canada's not one of those special cases in geography like China Proper/Han China's land area, Iran, the Roman Empire's default borders, or the USA core east of the Plains and definitely east of the Rockies: it's a bunch of different disconnected geographic areas stitched into one country, not even settled by merely one people like the initial core of the USA (re: Anglos) was for eventual gestation. And, as you noted, it doesn't even have an "event" that solidified the people into a coherent identity like the USA did or Rome (say Romulus or the formation of the Republic)/China (Qin Shi Huang uniting the Warring States)/Iran's (Achaemenid Empire) storied histories.
Canada will split someday in the future.
It's just when.
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