Most effective way to improve cantilever brakes
Pad quality makes a huge difference on rim brakes. While the spares you have may be better than the old pads, they're still going to be far worse than a good pad, especially in the wet.
I have a BSO with early V-brakes that take canti pads, and have been pleased with these Kool Stop Eagle 2 dual compound.  The dual compund (soft and hard) is key to all-weather performance. I use similar pads on my hybrid with proper V-brakes; both the stopping power and the wear are far better than with cheaper pads.   XLC make some dual compound pads that stop well but wear faster than those Kool-stop pads.
When setting up cantilever brakes,  pay particular attention to:

The pad clearances. Set them almost as close as possible to the rims at rest; true the wheels if you find you've got to back off the pads to avoid touching the rims at certain points in the rotation.
Setting up the straddle cable for maximum mechanical advantage (from about 3:50 in your Park Tools video).  This is about how hard the pads press the rims for a given hand pressure, but good mechanical advantage shortens the travel of the brakes, hence needing the pads to rest close to the rim.
Do make sure that the straddle cable can't drop onto the front tyre if the main cable snaps, especially if your tyres have plenty of tread - a mudguard or reflector bracket can prevent the wheel suddenly locking up from a dropped cable and causing an unlikely but serous accident.

With good pads, well adjusted, cantilever brakes are almost equal to V-brakes in stopping power; the main advantage of V brakes is their simplicity.
The Galaxy series has been around for a long time, so your bike could be quite old (on the other hand it could be from last year).  If it's an old model, not only could the pads have hardened with age, there's a slim chance the rims are steel which is rubbish for braking. I'm not sure it was ever sold with steel rims (and almost certainly not with steel rims and canti brakes), but on an old bike I'd check anyway.