I think I'll add some limoncello to keep it softer. How much do you think I should use?

And/or have a look at this article from Serious Eats, which recommends starting small (1 tbsp), taste, check texture, add more in small increments if you want and the sorbet holds up.
https://www.seriouseats.com/how-to-make-the-best-sorbet

I would add the juice of one lemon instead of two lemons and add maybe 1/3 cup of limoncello the first go around.