I should also mention, this is a long process. Using an rtx4080, it was almost 3 full months non-stop to convert my entire media library from mostly h264 -> h265.
and if you're looking to do software conversion you're easily looking at years, but considering how long most media servers will be up for, it might actually be worthwhile to aggressively automate that so it just runs in the background while you aren't looking. Also eats up additional CPU time which might be a benefit for someone.