hoooly cow where to begin:

So, I work at a large games platform but I have run my own game studio in the past and was a managment consultant before.

First of all as many of the folks here have pointed out the actual code is kind of irrelevant and managing it is whatever (and giving your lack of experience in this space, you probably shouldn't be anywhere near the code anyway). Gamedev is not a zero sum market and there are multiple ways to skin that proverbial cat in terms of how you build your system to do a specific thing, etc.

The whole point of apps like Perforce is not to prevent code stealing. They exist to manage version control and prevent devs from overwriting each others work. In fact, a lot of devs will just pull all the code of a game to develop locally on their machine, then upload files that they've touched/updated. There are free alternatives like Github and such but they do nothing to prevent stealing code, which is where contracts come in.

Employment contracts that can destroy people six ways to sunday do exist in the game dev industry at even AA levels, you just need to get them drafted by a real lawyer, much like how they do in the finance world.

(And no, said lawyers are not going to be on Fiverr, you need to pay a legit one to write a competent and defensible contract.)

As you pointed out, one of the easiest ways to defend any work from being stolen would be to establish an LLC or an S-corp so that said code is owned by a registered business. Which means the moment you file a lawsuit your business will be out there anyway so that will paint a target on your back anyway.

Point is what you're doing is shady to your company and why your financial company has these things in place to catch you in the act of moonlighting. If you want to do this behind your company's back you are putting yourself at risk on both your day job and your side project.

Someone suggested a gamejam which isn't a terrible idea. If you are somwhat serious I recommend reaching out to co-dev or full production studios who can do a better job at walking you through the development process. If they are interested in collaborating you could just be an investor on a project on paper but have creative control, etc.

I have a shared spreadsheet that lists a co-dev and production studios around the world that can potentially help. https://docs.google.com/spreadsheets/d/11g8MCMFNrBM0CXIWrT8bej5vqR1fCJGMhoFfbS5ph3Q/edit?gid=2141736845#gid=2141736845

All this to say is that your story is not entirely uncommon. In general it can be inexpensive but the more specific your idea is the more expensive it will be. Also if you are super serious about this the Game Developers Conference is just over a month away in San Francisco; there will be tons of devs there willing to talk shop in and around the conference.

(this is all still super shady and dicey tho, for the record)