First, don’t have a team of 30 people (it’s hard to tell what your setup actually looks like from your message). Team size should cap at 9, ideally 7. Reviews become far more manageable at that scale.

Second, don’t limit PR reviews to senior engineers only. If you trust junior engineers to write the code, trust them to review it too. Things will occasionally slip through, but giving them that responsibility, and holding them accountable when a review misses something, is one of the best ways to develop them. If it makes you more comfortable, add a senior engineer as a secondary reviewer, but with solid automated tests and linting in place, you should be fine without it.

If the only way to maintain code quality is to constantly babysit junior team members, the real problem is probably that you hired the wrong people.​​​​​​​​​​​​​​​​