unity - 3D game with Pre-rendered background

So a friend of mine and myself are starting to create our first game together. We descided on doing a 2.5D kind of game, so a static camera that moves with the player but cannot rotate.

I got the collisions object from my friend which in unity looks like this:

Now I'd like to create an overlay with a JPG image which is going to cover the entire viewport and is going to "be" the map itself. I want to use this JPG as a kind of background image and then hide the collisions so the player will walk into the collisions and stop there but can't actually see them. In stead, he will see the image with buildings on it.

How can I do this correctly? I've gotten as far as creating a second camera which will show me the image and then trying to create an Image (Create gameobject -> UI -> Image).

But I cannot find any way to continue on this.

Is this a good way to do this whole 2.5D kind of thing?

We want lightweight maps in our game and thought this was a good idea to go about this.

Edit

My friend and me are trying to create an isometric game like this. The reason behind this is that the amount of work for the map is going to be massively reduced if we can do it like this since he can then just work in photoshop, in which he's super good.

This was he does not have to bother with the 3d design in detail and can make the collision blocks much faster.

So does anyone know how to get this to work? Maybe someone has a partial guide on this because I cannot seem to find anyone doing it like this but I imagine that ie Commandos is made like this.

It would be much appreciated!