Linatural will likely need pH less than 5 to be most effective because of the potassium sorbate. It is comprised mainly of preservative boosters (propanediol and ethylhexylglycerol) so I doubt it would make a good preservation system.
Is Plantaserve phenoxyethanol + ethylhexylglycerol? If yes, then I think it is a good choice, although I’ve learned from this forum it is not the best for mold and yeast, so you may want to supplement it with something else to cover that.
Geogard Ultra has sodium benzoate, but the first ingredient is gluconolactone, which I’m not sure how effective of a preservative that is on its own. I think it is also just a booster. I think sodium benzoate will need a pH of 3 to be most effective, and practically ineffective at pH of 6.
Geogard ECT contains salicylic acid, which cannot be used by children three and under. Might not be a problem if your product is for adults only, but I’m also not sure how effective it is as a preservative.
Just my thoughts. Hopefully an expert will weigh in.