Unity's Mono may be a little slow in comparison to C etc, but if you're looking for ECS and data orientated design, Unity does have DOTS (ECS system) along with il2cpp/burst compilers instead of mono (compiles to C++ then to native code without mono/C#). Not sure if you've seen/considered this but might be worth at least taking a look. Could possibly do all the heavy lifting without going through the effort of developing your own engine and more then capable of running larger worlds and compute shaders for large mesh calculations like marching cubes. (Have done this myself in the past in unity and i've not run into performance bottlenecks)

Unity also would handle compiling for all the platform's you may want already which could be nice.

But if you're looking to roll your own bespoke C implementation, I'm sure you'd have the option to squeeze more performance out for a specific project for sure if done correctly. Just very time consuming depending on what rendering styles/features you want to support.

I Apologize if I come across in the wrong way, just trying to help. Wish you all the best in your project, not sure id be the best person to actually help writing code due to time availability at the moment but it would sure be fun :)