Higher % alcohol can result in wrong pH readings with certain glass electrodes.
However, ethanol does not really affect pH and you can as well set the pH before adding alcohol
By preference, use a ‘buffer’ which is soluble in 75% ethanol. However, there aren’t too many cosmetically suitable buffers which work around pH 5.75-6… there aren’t too many biologically compatible buffers in that range anyway. In addition to that, for most of those the charged forms aren’t soluble in such high amounts of ethanol.
A: it would be wise to use diisopropylamine or alternatively aminomethyl propanol aka AMP-95 as ethanol soluble base.
B: you’d now ‘only’ need to find a suitable buffering acid… alas, there aren’t too many to choose from… carbonate would be suitable at pH 6 upwards but you might run into difficulties working with dry ice as starting material. Acetic acid would be easier to handle but only works up to 5.6. Short to medium chain fatty such as caproic, capric, or caprylic acid would be better suited due to their good solubility in ethanol (in their acid form but also likely in their salt form). On the flip side, the product would reek like a horny goat. Hence, lauric acid and an ethanol soluble amine would be my pick (please note, such carboxylic acid buffers will only buffer to a pH of <6 and hence will only poorly protect a rise in pH whilst buffering well against pH lowering).
C: If you opt for the opposite combo of a buffering base with a strong acid, the acid part is fairly easy to cover but the base part… derivatives of imidazole, pyridine, and aniline come to mind. Sadly, most of the ones soluble in ethanol can’t be used if you value your customers health (and, again, their olfaction).