When writing my filtergraphs, I've found the following three "tricks" to be immensely helpful

1. Use an actual editor. Fish shell has a keybind that opens the current prompt in your $EDITOR, and saving+quitting returns to the editor. Makes multi line ffmpeg filters trivial. Doubly so if your editor has something like TabNine completions
2. If that's not enough, write the filter in a dedicated file, and use either editor automation or something like entr to run ffmpeg's graph2dot command and then graphviz to get a visualization of said graph. Helps ferret out bugs
3. Build up more complex graphs using either ffplay or mpv. You can add filters at runtime to mpv via the repl (press ~) and the vf add command