Volume or ml? Same thing, there are only two choices.
I've owned gram scales for decades and use them for weighing objects, powders, and other non-liquid substances.
Liquids are best measured by volume. Otherwise, their density must be factored into calculations to know 'how much' to use. An unnecessary complication. All of the recipes I've seen give measurements by percentage volume, or milliliters, or drops ... all volume, the way it should be. Measuring by drop count is very imprecise though, as different viscosity liquids and different dropper types produce different size drops.
I use syringes, pipettes, and beakers. Graduated cylinders might be useful too, but I don't presently own one.
High precision can be had with the smaller syringes and pipettes, like 0.01ml which is plenty precise enough even for highly concentrated flavorings. I have found pipettes to be a bit problematic with highly viscous liquids. But, with a 14 gauge needle on a syringe there's no problem.
Just another person's opinion. Thanks for letting me speak. :~)