Getting build times down is a difficult project because it's a cross functional task.

Like everything where you're involving a bunch of teams with no free space on their roadmaps, you have to be able to demonstrate that your team has done everything it can, and now the pressure needs to fall on the other teams. Like have a wiki or presentation to show how you have already trimmed the tests as much as possible, and the codebase is as small as possible, and you're not making 100 unnecessary docker layers, etc.

Thats your ammunition in the fight to get something like resizing the build instances on the other team's roadmaps and out of their budget.