Canadian spelling is possible but inconvenient
After concluding that Apple just doesn’t care about English speaking Canadians I did som experimentation.
Go to the “text replacement” settings and enter the American version as the “shortcut” and the Canadian version as the “phrase”.
Now it no longer “corrects” the Canadian spelling. It even seems to suggest the correct (Canadian) spelling as you type.
it is grossly inconvenient, but immensely satisfying to banish Ben Franklin’s spelling tyranny.