You are not entirely wrong in having left the sand alone and your tank does look healthy. Google: "Deep Sand Bed" which was great in theory, but difficult in practice. Nowadays, people just have deep sand beds in thier sump.
Yes. It is good practice to deep clean with a vacuum siphon. Some people do it monthly with the WC. Use a gravel vac that directs the water OUT of the tank and not back into the tank. But, go slow and thorough and follow up with a water change.
When you start siphoning, you are going to release major pockets of nutrients and anaerobic bacteria into the water column. Some corals may feed on this, some corals could get pissed, and some corals (the more delicate) could actually die from it. If this sand has been packed live for 6-7 years with nutrients, there are major colonies of cyano in there. I wouldn't know what to do. You might find Jimmy Hoffa.