Randolph, I think that's a good overview of what a tank is.
The first issue is she's got such a power spike at 2-3 you are almost hurting the team taking her solo where that passive can't be reliably procced. If she goes mid/top she can't expect to get a gank at those levels.
If she goes jungle the lanes she wants to gank may not be ready. And her jungle is terribly slow. She can't justify FF at all, and that means she's just going to fall behind further in this jungle meta.
So she'd need to get something for that tradeoff to justify this as a regular (or niche) thing and I don't see it.
Yes her passive works fine mid to late game, but right now things are focused on early game. Also the passive is a bit wonky in that it doesn't benefit her. The real benefit is for teammates.
For awhile I've been saying that one she applies it, she should at least get bonus AD against that target or maybe sustain on hit so there's at least a little self benefit. IMO it's design is parasitic. Name one other champ who's passive ability is not self-beneficial.
The biggest issue with her passive is that it is invisible power. Some champs like Sona I think are fine with invisible power. Sona is squishy, mute, and music based, you aren't supposed to see music and she shouldn't be drawing extra attention to herself. She doesn't have that flashy personality.
However with Leona you can't really feel or tell what that passive is doing. You can never notice it directly.
I think for a champ who is suppose to be in the middle of a fight, having a passive that's invisible (and requires coordination/communication to proc it) doesn't really fit with her theme. She's supposed to be super visible so the passive is against her nature.
Yes bruiser Leona either top or in the jungle is a thing, but it's just pretty rare. This is partly because she's kind of weaker when she doesn't build full tank.