> you really need the behaviors documented before you start screwing with it. It's very likely this thing has some pretty complex business logic that is absolutely critical to maintain.
Nothing I said suggested otherwise. Absolutely critical for whomever is doing a rewrite to understand everything they can about the application and the business, before writing a single line of code.
You sound frustrated that you've joined a company with an absolute stinker of a codebase, because you're confident you could deliver much better results having refactored it first. You're managing a group of people probably enormously under-productive because of the weight of the technical debt they're under. Every change takes months. It's riddled with hard-to-fix bugs. It's insecure. There are serious bus factor problems.
Many of us have been in this exact position before, multiple times. Many of us have seen somebody say "our only choice is a full rewrite" - some of us were the one making that decision. Many of us have seen that decision go disastrously wrong.
For me, the problem was my inability to do what I'm good at: write tests, write implementations that pass that test, etc. Every time I suggested doing something, somebody would have a reason why that would fail because of some unclear piece of the code. So rather than continuously getting blocked, I tried to step into my comfort zone of writing greenfield code. I built a working application that was a much nicer codebase, but it didn't match the original "spec" from customer expectations, so I spent months trying to adjust to that. I basically gave up managing the team because I was so busy writing the code. In the end, I left and the company threw away the rewritten code. They're still in business using the shitty old codebase, with the same development team working on it.
If you really want to do the rewrite, accept how massively risky and stressful it will be. The existing team will spend the whole team trying to prove you were wrong and they were right, so you need to get them to buy into that decision. You need to upskill them in order to apply the patterns you want. And you need to tease apart those bits of the codebase which are genuinely awful from those that for you are merely unfamiliar.
Nothing I said suggested otherwise. Absolutely critical for whomever is doing a rewrite to understand everything they can about the application and the business, before writing a single line of code.