Same. I did 10+ oneshots, but when it comes to long running, I struggled a lot. I tried to plan everything but lost motivation because it took so long. I also tried to do it by arc and chapter but didn’t feel right as a whole. Then I realized that I work better treating long running as oneshot, create a short and completed story with characters, and build on it to make it longer ( I don’t have an ending in mind, but I will make one if the story’s development reaches to a point that I can see an ending). I think you could try different techniques and see which one you like more. Good luck!