Discover doesn't actually download stuff, it's just a frontend for a package manager like apt, pacman, Flatpak or Snap. The package manager would need to support a speed limit feature.

Very good question and one that I can't answer. However I would check to see if your router supports QoS. If it does you might be able to ger the router to prioritize important data such as video playback, gaming, video calls etc. I've done that in my router and it made it so much easier to live with 10 Mbit.

On a limited connection, I'd advise not updating whilst watching a video. It's also possible to download a video and watch it whilst updating.

Beyond that it's not easy to answer - as I don't use discover, but also never do updates without paying attention, so that watching a video at the same time isn't something I'd contemplate.

Look into Bufferbloat and SQM.

One way is to replace your router with a router with good SQM (smart queue management). You can find recommended router at https://www.waveform.com/tools/bufferbloat

Switching to slow mirror can help (if such mirror exists)

You may or may not have a mechanism to limit bandwidth on the whole connection. It could be at the router level or possibly an option on your network card configuration. If you have the capability, setting a limit at 90% of your bandwidth should leave enough room to minimize the issues you're seeing.

Yeah as others have hinted at this is going to be easier with a router/network level feature.

I'm sure there's a way to do this by setting up a special virtual interface, limiting the bandwidth on that interface and then using iptables to redirect the apt traffic to that interface or something to that effect but I'm not a network engineer and I don't have the experience to help with something like that. There might also be a way to do this with cgroups to limit the bandwidth but again that's above my level of expertise.