I have read Plato's dialogues, Aristotles's "Nicomachean Ethics" and also Peter Adamson's two books about classical and hellenistic philosophies. Do I need to read primary texts of medieval philosophy or can I continue with Descartes?
Medieval philosophy deals with the relation between antique Greek philosophy and the Christian wordview. In short: The relation between revelation and reason.
These topics and a prototypic attempt to match both to understand our world can be found in the writing of Thomas Aquinas, e.g., in his textbook Summa Theologiae.
Medieval philosophy, named scholastic, develops or elaborates a series of technical metaphysical terms: Being, the term „good“, ens realissimum, god, salvation, sin, etc. which carry over to philosophers of the 17. century. Even Descartes, who makes a new start, uses some of these terms, often without further explanation.
Who has decided due to other reasons that the scholastic woldview and its ontology is outdated, can skip medieval philosophy on the first round. He/she can make a big jump and continue with Descartes and epistemology. In particular if one is interested in the new ideas introduced by Kant to epistemology, or even later during the philosophy of the 20th century due to the new approach of Karl Popper.
But in any case, I would come back later. And read some primary sources from medieval philosophy, e.g. the first quaestio with some of the first articulos from Aquina‘s Summa Theologiae.