Something that almost the entire population uses all day is only using 5%? Sounds incredibly efficient. Repacking fonts to be smaller sounds about as useful as plugging a hand crank generator in to the power point.
Solutions to the climate crisis can all be broken down into smaller and smaller parts — small enough to be ignored — or small enough to be easily fixed.
For web developers that 5% is our responsibility, and we have the means to change it in ways many people cannot.
As for fonts — yes I'm sure it would make a big impact. Font files are a decent chunk of average page weight.
You're pretty much guaranteed to make an immeasurably larger impact by spending your time researching and donating to climate charities, action groups, and politicians than you are doing any of the things you listed. Put your effort and money towards things that actually make a difference, not these tiny drops in a vast ocean.
