How deep can I #define?

Yes, you can. You may need to think about the syntax. Easiest would be to use one emoji per keyword. However you may still need to write function- and variable names in clear text.

As per Romens comment I tried it and you can also replace method names with emojis.

Just as a proof of concept, the following code compiles in visual studio 2019 with platform toolset v142.

#include <iostream>

#define  int

 () {
    std::cout << "I'm !";

    return 1;
}

 main() {
    ();
}

Or even more to include some of the comments:

#include <iostream>

#define  using
#define ‍ cout
#define  int
namespace  = std;
 ::‍;

 () {
    ‍ << "I'm";
    ::cout << "!";

    return 1;
}

 main() {
    ();
}

Also using is something else than #define. You will only need the latter.

Is it possible to #define preprocessors like #define #define

No, it is not possible to define macros to replace preprocessor directives. (Also, macros cannot expand into directives either).

or at least #define if

This is potentially possible. It depends on the compiler what input character encoding it supports. Emojis are not listed in the basic source character set specified by the language standard, so they might not exist in the character encoding used by the compiler.

Maybe with 'using'?

Emojis are equally allowed for using as they are for macros.

Note that any identifier could be an emoji (assuming they are supported in the first place) including functions, types and variables. Example:

struct  {};
struct  {};

int main() {
    {} == {};
}