Here is the video from ages ago that perfected my zabaglione step….I think it was the length of time she cooked it and the cooling step. Note how the next step (blending mascarpone, whipped heavy cream and zabaglione) is thick and dense. You want this so this filling doesn’t smoosh out the sides of the cake.
Where she added marsala, I use limoncello. And probably some lemon oil extract. Will have to see if my notes mention that. You’re going for a pretty intense flavor because the ladyfinger layer is neutral.