Same depth and led night as mine, good. I've got a radion led at 30% intensity (remember the maxspect you have isn't as powerful) so maybe you should up the intensity more than mine. Remember maxima clams love light so really I guess as high as you want the intensity is good, providing you don't bleach corals
I've had my clam on 1.5mm crushes coral/sand and had no problems. I've currently got it on the glass tank bottom as I took my sand out for other reasons.
As for pests, make sure you check for them as they like to get in and eat the clam from the underneath. That's why people like to put a clam on a flat rock as it will fix itself to the rock eventually. Never glue a clam down. Whilst you can keep the clam always on a flat surface so it doesn't fall over.
A.