hidden symbolisms that can be interpreted by whoever owns the correct decoding formula

I do not regard such process as interpretation - it's decoding, as you've already said. It is like the process while scientists try to decode some inscriptions on pillars or from manuscript ashes. AI is able to help in decoding. But the hereafter following process of interpretation has another quality.

Although you may make what you want of it, it was built as a riddle, as most paintings that were hung in Venice's walls in those days.

The riddle dimension is not the art dimension. I think both to be 'rectangular' i.e. independent from each ohter. The riddle dimension may have served as means to a pecuniary end, and in this regard as the economic base for the artist to produce his artwork. To solve the embedded riddle has almost nothing to do with getting involved to the art dimension. The art dimension secondo me lies beyond rationalism and scientism. It lies even beyond the intentions of the artist concerning the consciously known / knowable parts of intention and motivation.
I would not deny any artwork to have more than the art dimension, but I have in this other dimensions only interest in so far they can lead and guide me to the art dimension.
(This gives me the opportunity to remark that interest always has a goal and therefore needs pre-decisions.)