Someone hired you as a senior employee. Senior folks are expected to fix messes like what you describe.
Go to the right person and ask for the political capital and support to fix 2 specific things.
If you don't get a promise for that support or you don't see that support sustained, leave.
If you get that support, fix exactly those 2 things as quickly as possible. Then ask for the support to fix 2 more things. Etc.
You may want to walk into the right person's office with a list of 5 things to address and ask their input on selecting the right 2.