I want to suggest adding the ability to create sub folders inside custom effect stacks folder, to sort and organize them.
This is especially useful in cases where you want to apply effects that achieve a goal by being used together in several clips, as in the case of creating a frames grid by Transforming and Crop Edge effects.

yep i usually i use the copy paste-efefct to use same amount of effects on a new clip..
you can try to open a "suggestion" here : https://invent.kde.org/multimedia/kdenlive/-/issues (maybe filling with a visual "mockup" of your desired interface)