The leaderboards have essentially 0 effect on RNG, it's basically just an e-peen flex.
That said, I would try to at least knock out Difficulty 6s each week, because that gets you into the bracket where your weekly bundles "can" contain up to a DUO: Deborekah Earring. Obviously the RNG is abysmal, but technically it's still a chance, whereas 1-5 you can't get up to DUO.
Also, Black Shrine is HIGHLY dependent on your class. The reason those leaderboards are so filled has less to do with class popularity(although it is still a factor, sure)and everything to do with them being good at the bosses. Some classes have absolute dogshit kits for the bosses, whereas others it's like doing them on EZ mode. Just like so many other things in BDO, people literally have alts just for Black Shrine. Typically, if you have a low-stamina spammable i-frame and/or really good movement, you'll be able to do fine. If you don't, I hope you enjoy difficulty 1-5.