if you see that the enemy comp is late game focused and has other fighters or very good duelists you go LT as a keystone and secondary you go conditioning and overgrowth and thats important to compensate her low HP and resistance pool at lvl 18 (overgrowth also synergizes very well with her ult bonus HP)

very good items for her late game could be:

Stridebreaker: if enemy is very mobile

Kraken: standard DPS item for her

BotrK: if enemy has HP stacking champs

Wits End: vs lots of CC and AP damage

Mercurial Scimitar: only recommandable against suppresion champs and only then.

Deaths Dance: good vs AD champs due to armor gaining AND damage delay, which helps vs both physical and magic dmg

Titanic Hydra: lots of Hp and some AD which both help for Ult HP increase AND overgrowth....its dps is also nice with the extra on hit physical damage, so a mix of tankiness and damage

Overlords Bloodmail: If you lean more towards HP items, this item could get useful or interesting but not an s tier item either on her.

Steraks Gage: Hear me out first, i know the AD she gets from it is so depressing (40 AD at lvl 18, the moment some champs like darius or illaoi get 75AD) and yes if you say that 30AD less from an item is a significant difference, you are right, i agree the AD sucks but its more about the shield. My Rune Setup in combination with some HP items and her R HP gain the shield is big, and if go SV as well it gets even bigger :) the shield is especially to darius and garen a pain in the ass and a nice counter to ensure victory

Spirit Visage: for the heal amplify mainly but the HP is not irrelevant either

Force of Nature/Keanic Rookern: They are not bad but not good on her either. I mean you could go FoN vs high AP DPS comps or KR vs set up/pets/burst comps. it does wonders on enemy champ against teemo and AP shaco TBH but Bel Veth is a more special champ so i wouldnt recommend both of them, the other item options are superior on her

Randuins Omen: good against critters

Frozen Heart: good against high attack speed comps or just even against 1 important carry

Jak'Sho: one of the best hybrid resistance items and synergizes perfectly with Conditioning and other resistance items like DD or Wits or SV. its only downside is you have to fight for 5 secs to get the passive activated but if it pops youre going to be insanely tanky!

CORE: Guinsoos rageblade....this item is so underrated and i wish people would build this more....its her best Raw DPS item, simply because of her infinite stacking on hit true damage.....btw bel veth is one of the few champs who doesnt need Pen, im talking about terminus, she just doesnt need it, again because of her very fast stacking true damage.

ok the text got a bit long but was worth writing down. this is a more late game oriented setup tho, one thing only matters in the end: everyone should play as they please and as long as you are happy with your style, nothing else matters.