Out of all the items on the list (code reviews, CI, etc), it seems like test coverage would be the simplest to improve? Assuming it has a test suite at all, use one of the various test coverage scanners (simplecov, coverage.py, etc) and check the output for a file with a low % of lines covered. Then write one or more tests to cover the edge cases not yet tested.

It's a purely mechanical process unlike most of the other problems which depend on changing the behaviour of people in different departments and/or with more influence than you.