Would it be possible to automatically apply the filter immediately when placing down a splitter, such that it's possible to avoid having an item get stuck in the unused lane before being able to click the GUI or the hotkey?
Good idea! Released an update that adds this. Didn't enable the behavior on as default so check in mod settings. single-output – one output belt for splitter is enough to auto filter fully-defined – splitter needs two input belts and one output belt to auto filter