Ironically, my first thought was how quickly this might be able to be re-written, in a sane way, …with the help of an LLM.

I find that if you pay a lot of human-in-the-loop attention to the design phase, LLMs can produce perfectly decent stuff.

They’re also good at writing tests. Lock in the (non-broken) behavior of the old app with tests. Then use those tests to re-create the app, except with a talented engineer (you) guiding the design this time.