One of the biggest advantages of Godot (and certainly the biggest difference between Godot and Unity) is that Godot pushes you into the right direction in terms of clean code and project structure.
There is a long thread on github discussing the pros and cons of adding C# as a scripting language and among others I have written quite a bit to explain why I think that that would be a bad idea, so I am going to outline the arguments here and then just link to that thread so I don't have to type everything again.
While both Unity and C# have features that are designed to make mixing and matching of structures and patterns easier (see here for a longer explanation) Godot has features that support clean object oriented structures. One of these features is the combination of a very concise scripting language that is optimized for game-specific scripting and a very clean C++ interface that makes extending the engine very easy.
In my experience having a language that is optimized for library programming like C# as a scripting language has a bad effect on the code and the resulting gameplay. See here for a longer explanation. So I strongly suggest that you write gameplay specific code in GDScript and reusable code in C++. Most likely that will automatically fix your performance concerns, since performance critical things mostly happen in low-level code, not in gameplay code.
So right now I couldn't disagree with you more and I am also not able to follow your reasoning.
You may want to explain
- why you don't want to use C++
- what features GDScript is missing that would help in rapid development, i.e. that make development more "quick" rather than more "quick and dirty".
- how huge your combo system or your fsm is supposed to be that it's going to affect performance.
Also don't believe too much in rumors about where the engine is heading. The thread I linked was started because of the rumor that C# might be added to the engine at some point and so far several people have argued against it and only one has provided an argument in favor of C# (and not even a good one), so I don't really expect it to happen anymore. I mean it's going to be available as a side project for sure, since Godot has a very active open source community, but I am not afraid anymore that GDScript will suffer the same fate as Unity's Boo language.