Sounds like something that shouldn’t even take long to implement with steam input.

Shouldn't, but the thing is, you move all around the screen in that game. You can't make a mouse region for the aiming to get a direction unless the character stays at center or there's always a clear line through center to the reticle. When the character just moves all around like that, there's just no way to connect something to the direction from the character. You have to mouse aim on absolute position or use 8-way aiming on keys as your only character-relative option (what are we here, cavemen?).

The real problem is just that one of your critical gameplay actions is stopping shooting. Easily handled if you have a shoot button, like mouse, but controller only shoots on RS tilt, which means moving RS into the deadzone becomes a critical defensive action. You could work around that but you can't use Steam Input and controller-mode, so not even the option of having Steam use a big dead zone.

What you can do (I actually just got this to work!), is use Steam Input to mask the controller before starting the game, and then cursor key around through menus to the settings and toggle the "Enable Controller" button all on keyboard (hard to get it to mouse with a controller around). Once you get that saved and staying disabled, you can use a controller-free Steam Input config without the game switching to controller on its own.

So, I spoke too soon before. You actually should be able to play on a deck, because the trackpad will sub in nicely for mousing. Much better than the RS I have on a regular controller, but now I can at least play half-controller, half-mouse. Awkward, but better than either separately for me.