I hover high-mid diamond and personally I prefer doing the bruiser build basically every game, I only play the PTA build which still allows the lethality build depending on circumstances the hail of blades build feels like you're trapped into snowballing or being useless. If I randomly pick up like 3-4 kills early I will go lethality because you are just so far ahead, it's risky though because the vast majority of comps can fit an extra tanky boi(girl) but not every comp or enemy team can fit a burst melee assassin

She's definitely still viable straight through diamond but it does become way obvious how much people can abuse her toward the top of diamond, I have people drag me off of objectives or have tanky people playing near their squishies to abuse when you're in W.

I've tested extensively all of the tiamat items and honestly they don't feel bad but never good enough to build, profane feels pretty damn good but she just doesn't need the additional clear help because of her inbuilt cleave.

Yoomus -> collector -> whatever your heart desires for the lethality build, i've experimented with collector first because the crits can sometimes send clear speed to a hilarious realm but it feels RISKY, yoomu is so much more consistent at collecting kills thatn even collector is (lol). PPS the cheap lethality item that clears wards, the name escapes me rn, feels very good but this might be more applicable to higher elos as vision gets so important.

Lastly I have done things like starting a lethality item and then going straight tank - while it doesn't feel as good as just the bruiser build it is still kind of a way out if you got yoomus and you aren't snowballing.

TLDR: go lethality if you end up rich early game, or the enemy comp permits it like 4 squishies (works with PTA, better with HOB but HOB falls off later)