Flipping the House doesn't change enough. If the Senate is still Republican, Democrats in the House will do what they usually do: pass a whole bunch of bills named "The Protect Puppies and Kittens Act" with embedded poison pills Republicans absolutely will not swallow, so that opinion writers can flood the zone with articles about how Republicans don't want to protect puppies and kittens and how Democrats are so much more energetic about doing work in the House.
If the Senate flips too, it's unlikely to flip to a filibuster-proof Democratic majority.
If the Senate flips to a 60-vote Democratic majority or Democrats change the filibuster rule to simple majority, it's unlikely the combined House and Senate can override a presidential veto.
The pressure on Canada comes from the WH, and change there is still a little over 3 years away.
I doubt there will even be serious interest in Democratic held House and Senate to gin up an impeachment that has any prospect of succeeding. It would elevate Vance to the presidency with less than two years to complete Trump's term, which would mean Vance remains eligible for two more full terms.
The thing that would most benefit Canada is some kind of foreign affairs squirrel (or a bunch of them) that draws Trump's attention away from Canada for most of his remaining term.
The pressure to reduce defence spending (from inside the LPC and NDP party establishments and Canadians at large) is strong enough for things to be quietly shelved without fanfare without needing a less belligerent Trump.