One of your stated goals of the Sejuani rework was to make her feel like she's leading a band of melee "barbarians" into a fight. On this front you failed spectacularly. Sejuani feels like far less of a leader than she ever has before, before Sejuani would always be the first into a fight, engaging with her high impact ult and following up with her AOE. This no longer feels like the optimal way to play her however.
Sejuani's playstyle has shifted from that of a barbarian war leader who dives straight into fights to what can best be described as a 'manipulative tactician'. Since it's more optimal for her to allow a melee who's more adept at attacking than her to stack up her E than she is. It's optimal for Sejuani to follow after a Renekton, an Irelia, a Fiora or a Leona rather than for her to be the one who actually engages the fight. Those champions can quickly stack Sejuani's passive before Sej even fully enters the fight. This is what I mean when I say Sejuani feels like a tactician, when played optimally she's capitalising off of her teammates plays, not creating her own. You accidentally created a perfect Swain mechanic, at least thematically, but it's out of place as hell on Sejuani.
If you want Sejuani to actually feel like a leader she's going to have to, somewhat paradoxically perhaps lose her interaction with other champions. If Sej was the only one who could stack up her passive she'd have no reason to wait on her Irelia or Renekton to engage for her, in fact it would almost always be better for her to engage first, like it is at current. I also believe that it would feel a lot better from a gameplay perspective for both Sejuani and her enemies if Sej had better options for stacking E, I'd say on her Q and 1 stack for R impact + one stack for ult detonation would be appropriate but she lost her ally interaction. It would feel less bullshit if it was "well that Sejuani landed a great ult and followed it up with a lot of stuns" rather than "Renekton dashed at me and stunned me then Sejuani stunned me and had a high impact despite her Croc doing literally all the work".