Shuri's vengeful spirit arc was never about T'Challa. It was about her mother. She spent most of the movie in a state of perpetual anger over not being able to synthesize a cure for her brother quickly enough. She had even given up on it entirely despite it being a high priority for the kingdom. It's a perfectly normal part of the grieving process. If it's not dealt with in a healthy manner, it's gonna reach a climax, at which point you kinda just really want there to be someone or something to blame, so you can take that anger out on them. Her mother dying was that climax. Namor was that person she wished to direct her accumulated anger toward. That is why the vengeance thing didn't show up until the third act. It was allowed to pile up.

We can actually see Queen Ramonda in this stage of the mourning process as well, as evidenced by her absolutely wailing down on Okoye and then firing her, despite knowing she is still the most capable warrior they have, and knowing it isn't her fault that Shuri got kidnapped. This was the climax of her grief, and she chose to direct it toward Okoye.

A common thing between both Shuri and her mother's climaxes is that both of them can be traced back to Shuri's decision to go and get Riri before the fish people. Both characters blame themselves for the events that followed. Shuri likely felt that none of this would've happened if she didn't insist on going to get Riri, and Ramonda likely felt that none of this would've happened if she just put her foot down and prevented Shuri from putting herself at risk.

I agree with the rest though, but in fairness, that's something most of Marvel's more recent movies suffer from, so it's kinda low hanging fruit. I ended up liking the movie though. It's not a high bar, but it does stand as one of the better phase 4 properties.