At that length, you'll need to get something fiber optic like this. I don't remember which specific cable I got (I'm pretty sure it is the one I linked) but I got a 50ft one to go from where my PC is located to the 4k TV in the other room. It carries a 4k@120hz HDR signal no problem (though part of that equation will be your graphics card). Yes it will be pricey compared to a typical copper cable but once you go beyond, I want to say 20ft or so the signal gets pretty iffy even with signal boosters. Save your self the hassle and just get a good quality fiber optic DP cable.
I’ve tried a couple fiber hdmi 2.1 cables from “Vention” which works well but couldn’t find a DP equivalent.
I’m guessing this looks pretty decent from the reviews Fiber DP Cable
probably any displayport fiber optic extension or cable that advertises ”8k” ”dp1.4” and/or something like ”vr compatible”. look/search the reviews and questions for ”index” or ”valve” or ”vive pro”.
if you get one with 2 male ends, you will need an f-f passive adapter, like
or
one dp fiber optic cable i'd check out is this guy. i can't guarantee it'll work, but if i was putting money down on a bet, i'd bet on this cable.
or you can search for ”displayport fiber optic 8k 1.4” and nose around the reviews to see if anyone mentioned the valve index.
or you could search this subreddit for ”fiber” and your will likely find a few posts or more recommend fiber extensions that work for the person making the recommendation.