Rejoice!
What amounts to a systematic theology of David Pareus (d. 1622), a German reformed divine, has been found appended to an old edition of Zacharias Ursinus’s Commentary on the Heidelberg Catechism. It’s not precisely a systematic theology, but its close enough to count.
See if anything in it interests you:
Pareus, David at ‘Every Reformed Systematic Theology Online in English’ (RBO)