New Sejuani has the exact same problems you said you wanted to fix
Current Sej was (allegedly) problematic because a large portion of her power budget was tied into her ult, meaning the rest of her abilities had to be lackluster for it. Most Sej players were fine with it, though, because they enjoyed the high moment it offered and the satisfaction of landing a 4 or 5 person ult.
New Sej is the same problem magnified, but now with her new passive/E. It's now her primary source of CC, her highest source of damage (dealing 25% max HP late game), and other champions can build it up for her. As a result, all of her abilities including her ultimate feel just as bad to use if not worse. W is a shell of its former self, going from a hugely powerful DPS tool to one that does pathetic base damage and has laughable scaling. Q is the same. R is a joke, it's just a glorified Iceborn proc now. Even more laughable, because if you score a direct hit on someone and get the stun on them, they will be immune to both of the following slows AND the explosion damage. You will do more damage for whiffing your ult. Except unlike the current Sejuani and her ult, getting the stun and bonus damage doesn't even feel good or rewarding.
https://boards.pbe.leagueoflegends.com/en/c/champions-gameplay-feedback/MymFEVM9-sejuani-critique-and-suggestions my PBE thread offering more in-depth criticism of individual aspects