When developing core features in C++ the problem is that you would have to maintain your own Krita version which is kinda annoying since you can’t do C++ plugins. When your changes are generally useful to other people you could try making a merge request and it gets integrated into mainline Krita. For what you want to do, I don’t know if it’s possible since it mostly seems specific to just how you want it to have for yourself to work.
But what you could do is create the python bindings you feel are missing, this will probably have a good chance of being merged into Krita and then you (and others too) can use the extended Python API for the plugin.
UI wise a lot can be done with a Plug-In. I recommend taking a look at this
It’s like the dev tools of a web-browser and lets you inspect Kritas Qt visual tree and properties, which can help a lot when you want to add buttons or change behavior of certain UI elements.
As for changing how certain tools work (aside from button presses or something). I don’t think it can be done in Python, even with an extended Python API. It is as if you want to change how the engine of a car works compared to changing its gearing.