Successful Diet Soda Recipes?

My main goal with the Slushi is to produce various diet soda slushy drinks, as closely approximating their sugared, CO2 counterparts as possible. I went into this purchase expecting Ninja's diet cola recipe to be a great starting point, as Ninja generally has good recipes, but I had very poor performance.

I have found that to be consistent; allulose adds a lot of sweet, and lemon juice mostly makes it taste like lemon, not like the base liquid. For 24oz, 2 tbsp allulose is not sufficient, and for some reason, adding even 4 tbsp allulose to 24oz of drink from Sodastream's diet syrups results in it freezing solid on the drum (I don't have a refractometer to figure out what's going wrong here).

I have tried a number of different combinations, but I haven't found anything that results in a drinkable diet slushy. I'm not expecting it to come anywhere near perfect, but I'm not really have any success at all. I've seen users mention things they've tried in a few threads, but I wanted to try to collect them here in one place, and promote discussion so we can figure this out as a community.

Decent recipes