‘… sucks on PCM’ : Have you read this thread?–> Getting Oppo DSD output to the DS via I2S. In the beginning of that thread they discuss a board that looks identical to yours, and later in the thread (before post 114) they seem to solve the problems regarding PCM.
Regarding MiniDSP being transparent to the DS analog signal quality: I experience the same with my Lyngdorf preamplifier/crossover filter/room correction unit - the extra AD DA conversion seems to do very little harm to the DS signal.