Ammonium and potassium salts will have solubilities as high as
sodium salts. Polyvalent cations other than calcium or magnesium
may lead to low carboxylate solubilities, but these cations are expensive
or toxic. Anti solvents such as alcohols might be added to decrease
carboxylate solubilities, but the recovery of such anti solvents will
complicate the process too much.
Switching to a temperature where the solubility is low will be more feasible.