K2SO4 is not that soluble either.
I add this dry as well, typically after a water change only.
K+ can come from many sources, KH2PO4, KNO3, which are a lot more soluble and thus can be added as a solution or more frequently as desired/needed.
GH booster is nothing more than CaSO4, MgSO4, K2SO4 and was just DIY SeaChem Equilibrium which had been used to add GH to ur soft water and was the trend in the 1990's, we added lots of K+ to every tank.
In general, I keep about the same NO3 as I do K+ , maybe a little higher for K+.
Regards,
Tom Barr