Personally can’t justify many series in 4k, some of the ones I have only ever got SD releases (DVD at best) but there are a few I can justify 4K for. Mainly very cinematic shows such as The Mandelorian or The Last of Us. As long as they have subtitles in the other shows and are available in their best original release resolution it’s fine for me.

For example if the original Doctor Who series had a 4K release for it’s entirety it would probably be my entire server lol. 693 episodes in 480p is almost 300GB.