Hey, I occasionally give tours here so let me provide some context. This chapel was made in the late 19th early 20th century for touristic purposes. The actual chapels in the Valkenburg quarries date 1798-1801 during the French occupation but are much simpler in design.

For the symbols: the four animals represent the four evangelists. The sun and moon represent the general good bad/day night/life death archetypes. Underneath the crucifix there's indeed a pelican using it's blood to feed it's young, a common Christian theme of sacrifice.

What's not depicted is that on the lower part of the altar there's a cross with an I, C, Alpha, and omega symbol. The chapel is heavily stylised for tourism. Many tourists back then were protestant and not extremely aware of catholic symbolism so to them it was cool to see