HELP !! Delta firmware Guidelines

This is my first printer to build from scratch, and my first delta printer , i have a problem using marlin to make a custom firmware for my delta printer , and it has been a while since i ever dealt with simple programming, there are very few helping forums that are interested in delta printers, and too bad i have no specific model to memic, so i made it custom from scratch
I really do not need where to start , I began with a biqu kossel dimensions , but changed the board to skr 1.3 , switched drivers to tmc 2130 , and bought the motors, fish eye and arm lengths that were available ,
so by time i realised that i have made a different version of the product, and have no clew how to edit firmware , and even biqu has discontinued their support for their product long ago for problems it showed , so i have no updated firmware , nor the knowledge to make one , specially for a delta , i feel lost
I am asking for if
1- there is any educational videos that goes step by step to make a custom firmware for a delta printer , i mean really for dummies level , i feel like a child working through this high level programming and delta caliberations
2- if there are any professional services for firmware making , i have seen fiverr, but find no professional service worth , any recommendations

will appreciate your help and answers guys thanks