This almost sounds like a Core-XY printer. Is that what you are doing?
Start by looking at how Delta, Scara and Core are handled differently than Cartesian. You will start to see patterns. And depending on the kinematics, the places those different movements hook into the code base is different.
Spend some time (a few days) understanding why they hook in at different levels.
What school are you at?
That's the theory, now the question: Where is the best place in Marlin to implement that?
Start by looking at how Delta, Scara and Core are handled differently than Cartesian. You will start to see patterns. And depending on the kinematics, the places those different movements hook into the code base is different.
Spend some time (a few days) understanding why they hook in at different levels.
What school are you at?