In general, the differences between Tier 2 and Tier 1 are:
1. Get tests running on every commit.
2. Gate building on those tests passing.
The first is a technical/money issue, the latter is a social issue. That is, getting all tests working is technical, being able to afford running them on every commit is monetary, but gating builds on failures on a platform is an issue of "who is responsible for making sure things stay passing."
Given that we run these tests on every commit, in some sense, it would be the PR author's job, but if they get stuck, we need someone who's able to help them fix that. That is the hardest part of moving from tier 2 to tier 1 in my personal opinion. Who is that person? What happens if they drop off the project?