I live in a high humidity area, and taught workshops for kids, I prefer to use an emulsifier rather than a solubiliser as I have issues with the solubilised kind. Have had luck with just water, but much better and never fails (esp. important when doing workshops with kids) so I use an emulsifier.
The spinning effect of a bath bomb depends on how the gases (CO₂) from the citric acid and baking soda reaction escape, as well as how foam and surfactants interact. Yep, I’d wager your SLSa is likely contributing to the excess foam. While 2.6% SLSa is within reasonable limits, it could be overpowering when combined with other factors, such as the tightness of the press or the amount of binding agent used. I’d try reducing it and see if it improves.
And it could also be the amount of tapioca starch you are using. Your ratio between Na Bicarb and Citric seems okay, but your tapioca might be a bit high and will effect spin. I’d personally reduce it, it’s just filler and really isn’t a necessary add.
PS80 doesn’t directly dampen spread but too much can add viscosity to the bath water, so will slow the fizz or cause clumping. Although your input is low, so I personally don’t think this would be too much of an issue.
But I find you need some air in them to spin, some people put pins etc to add airholes, I don’t both but I don’t pack tightly. The more densely it is packed the more likely it will trap air and not spin, when packing play around with how much you compress, if you are using a pneumonic press you are more likely to be packing tightly, but you can alter this. Just play around with it, and work out what works best for you.
You could add some PEG40-HCO but you would want to reduce your PS80 I would think, mix your PEG40 with your PS80 and water mix to homogenous before adding to dry phase.