I admire your dedication to this game.

I remember last year for 2 to 3 months, I was swimming in deep C language ( I was-and still am- a novice), initially starting to build the solver. From there, I moved on to create a sudoku generator, and finally to a sudoku game program, all in C. Perhaps a right way forward for you is to go backwards. While you know what you want, get a solved riddle, and then with your condition in mind, move backwards, taking out the numbers that fall into that condition, and thus create an interesting, for you at least, riddle.

Finally, when you have walked the algorithm yourself, it is time to code.

It seems a great but fascinating task.

I always think that in every impossible and unsolvable riddle, there is a logical way forward that escapes me. And while reductio ad absurdum is an inferior, weak and low-level logical method, it remains a logical method nonetheless, that in the end of the day, gets the job done.