Hi there,
I'm Jansky, my peak this season (12) has been 700LP Grandmaster by mostly onetricking toplane Belveth and keeping a 60% Winrate in over 400 games. I mostly play the champ top because I am a toplane main and i am bad at jungle but I still like the champion. I found a way to make use of her, even if you stack your passive slowly or way less and that is building her as a tanky bruiser, that does not get oneshotted every fight but still has damage to dive and kill multiple targets in teamfights. I hope you can make use of this guide and if so, feel free to follow my Youtube (https://www.youtube.com/channel/UCxRQ5FwD1t7Hd1oIzyv4CqA)
or my Twitch (https://www.twitch.tv/janskylol) for some Belveth gameplay.
Best of luck and enjoy :)

In my time of playing Bel'Veth I always disliked getting oneshot after minute 20 or such and to literally even get punished for picking up my ults in teamfight, because you lose damage not attacking. With this build you obviously lose damage but you still carry enough to fight most champions in a 1v1 and if you are ahead you can even turn fights with numbers in enemies favor. To sum it up: You trade some damage for being alot tankier. Also, this will be a rather general aspect but learning to play Belveth toplane takes time. Alot of time. In my first lets say 15 games, I've lost about 14 I guess and I literally unintentionally ran it down every game because the champions feels different to play than anything else in my opionion.Also I've been researching the "ideal itembuild" for about 250-300 games and I still baven't figured out, what would be the optimal go to but at my current point, the Bruiser build with Titanic, Heartsteel and Stoneplate is my best go to.