Use SD feeds instead of HD? Some providers provide one of each for main channels

This is not possible.
The only option i know of and as stated below is to use a Provider that offers lower quality stream, which means lower bandwidth.
Some providers have both high and low quality channels.

I have 10/3 internet and can watch 1080 streams for hours without buffering. Now in prime time it buffers (sometimes to the point of things being unwatchable) but I kind of expect it. Are you sure it's not your IPTV provider causing the issues?

I could be totally wrong about this but in Emby and Plex there is a setting to reduce the bitrate.

No, you're stupid for asking this as you have absolutely no control over what your provider sends you.

Just get a provider with SD feeds. They do exist.