Where to start kernel hacking?

That is generally a terrible idea which will lead to an exponentially increased complexity

It doesn’t. As long as you keep them as separate modules with well define API.
(hint: Unix files and pipes are anything but simple and well defined).
In fact my approach reduces complexity – the Unix style filesystem emulation.
Since there are no intermediate files or any filesystem access inside the modules themselves.

For example, my NCM replacement for C preprocessor doesn’t even know about files. Same way, NRM doesn’t know about files either (it has a single virtual file which it repeatedly rewrites). It is the job of the user to define filesystem access. I used NCM for three project:
1. to preprocess assembly..
2. to preprocess NewC code.
3. to preprocess Symta (the Lisp system I work on).

The NewC and assembly code gets preprocessed with NCM built as a separate utility.
Symta uses NCM as a single header library, since it gets called on each eval.
For example, when I run a script or open a console in my video game.

Moreover, I want to take NRM’s file manager (the one managing rewrites to file), move it into a separate module, which could be shared with NCM, which I will use a base for C compiler or well an LZW unpacker, because struct FILE is just an opaque Unix thingie, disallowing me to define my own, since it is was expected that one will use Unix itself to help with input redirection.

Same issue with grep/sed/pcre. They are separate utilities.

If you like such approach, then, by all means, use Unix.

Personally, not a fan, so hope one day completely moving to RISC OS.

I wont cry if it will be 5% slower than Linux or wont run your latest systemd.

asasm is part of the GCC toolchain. If you start moving into this direction, you will bring in the rest of the GNU stuff and the GPL with it. At this point you may as well rewrite everything into Rust. I heard they are now doing that to Linux kernel, right after they added C++ support.

They aren’t rewriting anything, just adding support for kernel modules written in Rust, and I’m pretty sure that in 5 years time when all the hype has died down and there are no more than one or two Rust modules which are unmaintained, it will all be ripped out again.