Modding Bedrock Edition is not possible, or at least not the way how Java edition works.
Java allows dynamic loading of classes and code, as well as ASM and runtime injection. Without these assets, Minecraft cannot be modded.
If you would like to make a new game, make a new game. Minecraft isn't built to be modded.
Personally, according to the OP's description (which appears to be a block-based game), I would say maybe pre-made engines like Unity is not the way to go. Unity is not that flexible in that the user cannot control the rendering of mesh. AFAIK Unity does not support occlusion culling with dynamic components, which is crucial in a block-based game to prevent the rendering of unseen/obstructed faces. Better performance may be achieved with Unity's new entity component system, but I would still see the lack of rendering optimization as the major problem. Personally, I would recommend to write your own engine (with OpenGL or something) in this case.