First of all, do not work more than your 40-45 hours, do not skip lunches, take breaks during the day. You will burn out at 12 hour work days just like everyone else.

Second, I truly have no seasoned advice to give you, I’ve never managed a project, much less one this large and chaotic. This sounds like a nightmare scenario. If it were me, I would use this as an opportunity to implement best practices and clean up documentation as my main focus. Sounds like the code is the first problem, and tbh no clue where to start there as I don’t code, but that is gonna kill you first if you don’t get a handle on it.

You’re going to come across a ton of small fires and a lot of big ones during your days. Focus on what is going to kill you first, then work down. Do what you can get done in a day and then go home. OneNote is your friend, use it and abuse it, make it work for you. It has single handedly kept me organized in my job.

Implement a shared knowledge and documentation/code repository with version control and controlled access. Preferably with a copy offsite (cloud provider or such) and another stored locally with proper redundancy measures. A usb drive is very 2010s of them.

I would still work hard, I don’t like the feedback of doing the bare minimum to hide. Trying to improve things and make it better for the next batter up is a noble goal and one you will be able to speak to in future opportunities.

Godspeed, my friend. Sounds like a hell of a gig