What should I do with my leftover bandwidth?

A few years back, I was in a similar situation and found BOINC(https://boinc.berkeley.edu/) to be a great way to contribute. It's a platform that lets you support various scientific research projects by sharing your computational power and bandwidth. However, it's worth noting that BOINC might tends to be more CPU/GPU intensive rather than bandwidth-heavy(I'm not entirely sure about its bandwidth usage)

Host a CORS proxy server. Maybe like this one: https://kraker-remote.vercel.app

There used to be several options available a couple of years ago but almost all of them have gone offline. It would nice to have more options again. Mine is a good baseline to go with. Runs with Nodejs and has no dependencies but is quite powerful.

Find the source code here: https://github.com/8chanAnon/Kraker-Local-Proxy-Server

You can promote your proxy server here: https://gist.github.com/jimmywarting/ac1be6ea0297c16c477e17f...

I have multiple water spigots in my home, I never have them all open all the way. I don't use all the electricity available to me either. I don't breathe all the air that surrounds me.

Bandwidth isn't a perishable resource. If you don't use it then it's available to other people. I get the impression you overpay for bandwidth capacity you don't need and a static IPv4 address. You could just pay for what you need, free up the static IP, and then not have to signal like this.

You could be a CDN (Content Delivery Network) and act as a point-of-presence (PoP) for the country you're in. Mirror Linux distro ISOs, offer the ability for netizens to download offline versions of Wikipedia which are over ~50GB sans images.

Wrong answers: Seed Metallica? Become a Tor exit node? Join a vigilante botnet?

Better answers: Run your own vpn or Owncloud instance or run your own Matrix/Jitsi server

a non-exit tor node.