Erythritol isn't really a valid sugar substitute. At least, not on it's own- and not in something that requires the texture or the chemical properties of sugar.
The alcohol in the limoncello gives you some freezing point depression, so that helps, but, you're going to need some sugary bulk.
If you were in the U.S., I'd tell you to buy allulose. I'm not head over heals in love with allulose, but, it's the closest thing you'll find to sugar without having any obvious down sides- at least, not as of today.
Are you making this for yourself or serving it to others? Inulin can be laxating, but, then, so is erythritol (for some). As long as you keep the serving size small, though, the inulin should be tolerable- and actually adds some health benefits in the form of a prebiotic.
If I were making this, I might start with 2.5DL inulin, .5DL erythritol, .5DL xylitol for the sugar component. For 5DL water, that's about as much erythritol and xylitol as you're going to be able to work with while avoiding issues with crystallization. I would make the mix, chill it in the fridge overnight- maybe even 2 nights, and see if the sweeteners start to crystallize. If they do, you'll want to back down on the erythritol and xylitol even more. Inulin is pretty powdery, so 2.5DL isn't much, so you can definitely increase the inulin for better texture, but, remember it's propensity for laxation.
If you have dogs, forget the xylitol, though.
That's the textural aspects of sugar. As far as sweetness goes... Lemon requires a load of sweetening- and stevia isn't up the task- at all. You're going to need at least one other high intensity sweetener in the mix. Sucralose will help considerably, but I don't think even sucralose and stevia can rise to the task of countering the sourness of the lemon. Maybe add aspartame as well.
