Broad spectrum preservatives for 'green' marketing

Hello all! 

I'd like to ask for some input on a bit of a dilemma I have. I'm trying to find a good preservative system for a  cold-process serum (simplified formula below) with a pH of 5.5~6. The pH is required for stability but limits my preservative options. 

The tricky part is that the formula needs to be made with only EWG grade 1 or 2 ingredients. As much as I wish for the Skin Deep pseudoscience trend to die down, EWG unfortunately has a lot of influence over buyer perception. 

I've tried silver citrate but had it fail miserably in small-scale microbiology testing. I've currently added potassium sorbate and pentylene glycol, but I'm wondering what may help round out the spectrum. Potassium benzoate, glyceryl caprylate, caprylhydroxamic acid, would any of these do the trick?

water 75%
niacinamide, panthenol, sodium ascorbyl phosphate 13%
polymeric emulsifier 1%
oils and esters, 8%
arginine (for pH) 1%
pentylene glycol 1%
potassium sorbate 0.3%
Disodium EDTA 0.1%

Thank you!

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