Okay, I stumbled across this one on the Blender forums. Mitsuba. Supports Collada and OBJ. Documentation is a little sketchy, but the OpenGL realtime preview/scene navigation is pricelss.

Personally if I was looking for a free renderer I would use something like Carrara that's been given away on various CG Magazine cover discs. You then get an easy to use 3D program to apply materials and a reasonable GI renderer.

3DKiwi

Another cool solution can be to use the new free 3DCOATPrint!
https://pilgway.com/product/3dcoatprint

You can use the few existing Shaders but also create your owns! ;) See the video folowing!
And of course mix your Moi3D objects exported with the 3dCoat objects modelized or free downloaded!
Accept NGons...for the OBJ format!



Press this key and above it for call the Render and settings - any size allowed!

I've been using 3D Coat for close to 10 years. I'm now use 3D Coat 2022. I've not looked at their free version.
However 3D Coat requires your models be UV mapped and if they are not UV mapped, 3DC will auto map your model.

However 3DC really prefers quads for automapping. I've had issues with tris and n-gons in 3DC.

II's PBR based shaders are incredible but again require UVs.