Not sure about how helpful this is. I use a GTX 1070 in my server, but I most of the time have only one concurrent stream. It handles everything (only up to HEVC obviously) flawlessy for now. It’s a card with a zero db mode and I’ve never seen the fans spinning since it’s in there, so the load on the GPU even when transcoding can’t be very high
Since you already got a lot of ELI5s, here is a basic to-do to get you up and running. From my experience, since I use the exact same setup as you describe.
Ben phelp’s homepage (you can find it on github)