Now you can find the same 4K video from few GBs to a hundred GBs, and I am wondering: where to stop? With music there is a similar phenomenon by which after a certain bitrate it becomes an esoteric art to detect improvements. So, what is your “very good enough” bitrate for 4K videos?
At the end of the day it’s about what you like, what is available, and how much space you got.
My rule of thumb has been 8GB per hour of content for 4K (I don’t remember where I heard of got this from, so at the end of the day, this is just some arbitrary number). I usually stick to x265 encodes and so far this had been good enough for me. Some prefer the best (untouched remux), but like you mention, these files are huge. Even though I have many drives, I dont want each movie being 70+GB per file. Sometimes I break my rule of thumb and do get “higher quality” (that isn’t a remux). I think the biggest file I have is around 50GB for a x265 2160p encode of a movie where a certain king returns. As with everything, there are exceptions. Just do what you want.
This has been good enough for me. Obviously, the bitrate of audio matters and its format. That, in and of itself, is a whole other issue ( lossless or not, channels, etc…)