I am in a platform team that has had many senior-level folks since the start. There are many great things already present for our team's productivity like:
I am the junior-most in my team and want to target for a Senior Engineer role. One way of approaching this is by trying to improve the life of my fellow teammates. But I do not know what I should be adding. Because I do not know what more would be needed.
I know this may be a flawed way of thinking. I think it's better if I just keep on doing tiny improvements every day. But in terms of impact, I feel adding some tools / taking initiative here will help. I want to know what can I add to the team's arsenal which can also help me in my journey to Senior Level.
I love that you are trying help address major issues for your team and are being ambitious about your own growth.
If you are most junior member of your team, the priority in my opinion isn't so much to chart your own path, but to learn from the other folks on your team.
One thing to note is that while there are many engineering practices that are helpful, few are universally helpful. Even "code quality" can have varying amount of importance depending on the stage of the product / team. So it's important to understand the situation well before proposing changes to the current way things are done.
As such, I would try to really understand the perspectives of more senior folks on your team and ask them what they find really annoying, and start addressing some of these issues.
Having said, if there's one area you might be able to help with is onboarding, since you probably onboarded the most recently. Maybe you can improve documentation or start a mentorship program?