This is very complicated to just give an answer, because:
It varies greatly based on the content. Animated compresses vastly differently than an action movie.

Varies greatly based on encoder. NEVC vs CPU etc

Varies greatly based on encoder options. I.e. -b:v -minrate -maxrate vs -rc vbr -qmin -qmaxcq values, etc

Varies greatly based on who is watching, the TV they use and their tolerance and experience.

Savings are greater at 4k than 1080p. But once you start adding HDR into the mix, you’re in a whole new world.

Even the people with very discerning eyes can’t agree on everything related to this topic. Wish I could just tell you do x… but you’ll have to test various methods and determine what you are happy with.
or, if you just want some space savings… use some default setting that cuts it in 1/2 and forget about it.

Really depends on the gear. Some of it you can, some you can’t.

Check IPMI and see if you can adjust it there. For mine, I use the IPMI plugin that you can get for Unraid. So some OS’s do have solutions. But depends on your OS and hardware.

You can replace fans, usually. But need to make sure you still have enough airflow for the equipment.

I did replace my server grade fans with the Noctua Industrial. They are still a bit loud for being in the same room, but quiet one room over. Where before you could hear it in any room of the house.
My goal was quiet one room over.

Sure, I have hit 10TB/mo. I’m making sure I get my money’s worth for paying $10 to have unlimited with comcast.

I think in general they have lower RPM and run a bit cooler and use a little less power. That usually comes with a bit less performance.

But I’m hooked on the WD Ultrastar series. Server Grade and fast. Also has low power usage, at full tilt, mine use less than 10w each. I’m running 20 hc530’s and been rock solid.