I. Stabilizing the codebase
When I inherited the project, it was immediately obvious that it had switched developer hands several times with some taking less care than they should have. There were a lot of features that had been added without documentation and the Git repo had stopped being maintained. It was clear that I needed to stabilize things and get a clear understanding of how the site worked.
After a couple months, I had the site in much better working order with an up to date Git repo, commented code and a good understanding of how the site was put together. This foundational work helped solve a lot of issues that came up later.