I don't think buffing Sejuani through her damage is a good idea
Riot tried this before. It did not turn out well.
Furthermore, the champion Sejuani is most similar to is Amumu, who has two cc abilities on his kit and two abilities that ONLY deal damage while Sejuani has 3 abilities with powerful cc on them and her Q and R matchups deal strictly superior forms of cc to Amumu's. Superior damage output should always end up being what distinguishes Amumu from Sejuani for these reasons and Riot should look to improve Sejuani by bringing out her AoE cc power (maybe by giving back some of the duration on permafrost).
If she has trouble on her clear speed then I think a monster-only damage boost is a better idea because Sejuani being able to deal a lot of damage while clogging up enemies and having some of the best base defensive stats in the game is a combination that we have seen before and it had to be stopped.