Thank you so much for visiting this thread. I especially appreciate it as you always know what you are talking about, in particular as art goes. This time is no different of course.
To be true, I had not thought much about the artists' impossibility, back in the times of Bosch, to completely freely express themselves for the reasons you expound; but it of course makes all the sense in the world. Not to mention the ommipresent Inquisition which, if I am not mistaken, was already active at those times and might have found Bosch's paintings heretical. Fortunately enough, he and his art found good and powerful protectors both while still alive and after he was dead, like King Phillip of Spain (who ruled over the Netherlands at the time), a great admirer and collectionist of Bosch's works who, in the words of Father Sigüenza, "would have never let them enter his rooms and the Monastery sacristy had he ever seen any traces of heresy... "
Yet as you say, art was so important in bringing the Christian messages to the lucky viewers. In that sense, it was perhaps fulfilling its highest mission on Earth at the time, and to make it effectively the best means was, like in all traditional teaching, the use of symbols.
I, like you, consider the art of Bosch's times to be a strong (and most authentic) art, much stronger and authentic than that of the seventeenth century. And we are certainly most fortunate to be viewing it now.
Luis Miguel Goitizolo