Aquinas offers a clear account of the epistemic priority of Aristotelian substance in his Commentary on the Metaphysics bk. 7 l. 1:
- (1) It is evident too that substance is first in the order of knowing, for that is first in the order of knowing which is better known and explains a thing better. Now each thing is better known when its substance is known rather than when its quality or quantity is known; for we think we know each thing best when we know what man is or what fire is, rather than when we know of what sort it is or how much it is or where it is or when we know it according to any of the other categories. […]
The point is that we know a thing better when we know its substance than when we know only such 'categorial' features as place and position, quality and quantity. To know the substance of X is to have a more fundamental, explanatory knowledge of X than to know merely such features.
To use your own language, it isn't the case that 'a substance can be or must be known without or before knowing its properties'; the claim is only that to know X's substance is to have a more fundamental, explanatory knowledge of it than we have when we know only its properties.
On the basis of the Meditations, it seems that for Descartes knowledge of substance has a similar priority. My essential nature is to be a thinking being, i.e. to be a thinking, non-extended substance as distinct from a material, non-thinking substance. You have a more fundamental, explanatory knowledge of me when you know that I am a thinking substance than when you know anything else about me.