I'd suggest taking a look at the excellent book "Ice Cream" by Goff and Hartel (ISBN: 978-1-4614-6095-4). It covers all aspects of making ice cream commercially, and is very thorough. Chapter 8 is titled "Soft-Frozen Dairy Desserts" and page 252 has a table of example formulas for soft-serve ice creams.
For example, one column of the table shows the percentages to use for 6% milk fat:
Milk fat: 6.0%MSNF: 12.5%Sugar: 12.0%CSS: 4.0%S/E: 0.4%TS: 34.9%(MSNF: milk solids-not-fat, CSS: corn syrup solids, S/E: stabilizer plus emulsifier, TS: total solids.)
To take a guess at your actual problem, I'd say if it's not soft enough, you might try lowering the MSNF and xanthan gum first.