Unfortunately, grumbler_jones is correct. I tried Unity myself. It's a no go. And RPGMaker, for that matter. Which is a shame. There's a couple of physics engines I'd like to try, like GMod, but my hopes aren't high for that, either.

Even worse, oftentimes you'll be dependent on using models, and as you can imagine, modelling software to model and manipulate and animate meshes is not accessible either. It's like they didn't even think about us as a market . I suppose in their cases they can be forgiven a little, we're not the typical demographic. But I'd have loved to have got somewhere with it.

Somewhere that holds up some hope though is that a lot of Unity stuff can be done in Visual Studio, which is, for the most part accessible. I'm not sure whether the Unity parts themselves though are, or how much it lets you do without the inaccessible Unity IDE.

Another option, that isn't an engine, per se, but allows you to programatically describe and draw your meshes on screen, is Three.JS. I had some success with drawing some things on screen with that. It's not a gaming engine itself, maybe someone's built a gaming engine around that?

I recall that some people in [this] thread managed to get something working with Unity outside the IDE environment. Its also possible to purchase 3D models for use such as off the unity store or elsewhere, but accessible 3D modelling software is still a work in progress. At the moment you could use OpenSCAD to script a 3D model, then export it as an STL file and view it with the AudiMesh3D tool I developed, although there is no animating or texturing available so that would have to be done programatically. Conversely, you could take a 2D approach like earlier 3D games such as Doom and use sprites instead using an available paint tool such as BrushTone, TactileView, or SVGDraw01.

Other than that as others have already mentioned there currently isn't much available without picking up a code book.

There's nothing wrong with using stock assets, especially for testing or development purposes, but if originality is a concern for a more polished release artists often do custom work. I did mention there were [other] [sources], or contacting artists directly for custom work, although custom assets are generally more expensive than stock assets because of their exclusivity which if your developing for educational, experimental, or placeholder purposes might not be worth the upfront cost if the project ends up not going anywhere, even then you can always upgrade and replace those assets later. The point being you don't need to do any 3D modelling yourself to get 3D models.

As for Unity, the thread I linked to involves using a script template to develop with Unity without the Unity editor, theres a link to it in there. This is more like using Unity like a 3rd party library you import when scripting, like Panda3D or Pyglet, which granted doesn't provide the more convient interface of the IDE, but it can be used to develop games with Unity. Some relevant snippets: