Serious-Sam-Android VS OpenSceneGraph

You can try source engine ports like xash3d diii4a Serious Sam Freedoom OpenLara Quad Touch and Source Engine

serious sam TSE (Tricky to install)

Serious Sam: Serious Sam Android

For Serious Sam, you may want to play with the Android port instead - https://github.com/aarcangeli/Serious-Sam-Android/releases

I started in 2001 with OpenSceneGraph which made extensive use of intrusive pointers. This was 10 years before C++11 note. That codebase continues to be relevant and as performing as ever.

browse github and try to contribute. I started with OpenSceneGraph

I have been using OpenSceneGraph for over 20 years. It is used in the military a lot and I myself have seen it used to simulate war scenarios with realtime data aboard of a command-and-control center of a battleship.

Does http://www.openscenegraph.org/ fit?

Speed Dreams: It is a fork of TORCS, and was born as the need to include many more functions to the latter. In Speed Dreams the menus would be completely redesigned, adding many more options and making them much more intuitive; the game acquired dynamic time, improvements in reflections, career mode, a new simulation mode and multi threading. Over time many more options were implemented, such as local multiplayer mode, Force Feedback, and of course more and better cars and tracks, a new and more powerfull graphic engine (OpenSceneGraph) among other things, making it a much more complete game than its predecessor. They recently they have presented version 2.2.3. You can also install it in AppImage and Flatpak format.

Here is an example: https://github.com/openscenegraph/OpenSceneGraph/blob/master/examples/osgsimplegl3/osgsimplegl3.cpp

openscenegraph (mainly focuses on graphics, used by openmw for example)

Is not little, is very complex open source racing simulator. There are a lot of cars, categories, tracks. The physics has a lot of work . About the graphics, the devs are working on change the graphic engine to OpenSceneGraph, that actually works, but needs much more polishing. You can see his quality on this video: https://youtu.be/4daTgcxEWRg

I love Godot for a complete game engine. For just the renderer you could consider http://www.openscenegraph.org