If that tank above was mine, or Tusi's large tank this is how I'd make an 8 mo slow action occur all at once on a Saturday with no recycling: a parted out cleaning, no section work.
The hardest part is sourcing a full water change. Some forego that due to work, and siphon/store current water separately before tearing into the bed.
That's either a lot of rubbermaid trash cans of new mixed water or water truck delivery from mobile LFS service unit, full WC is the ideal, using old water is a lesser option for the ends in sight
After water prep, drain entire tank to zero house fish separately of all live rock only by themselves
House corals that can be moved separate ideally
Live rock: it has detritus retention since it sat on that bed as long, it must be housed in this procedure separate of all life like fish and corals as it is rinsed and checked for detritus pumping out of it while in storage container. Be exporting that; don't let it sit and rot that's the first recycle risk in the procedure--only detritus causes a cycle...work correctly with detritus, have no cycle. Ever
If items are accreted to rock, we can work with that but we need to be cleaning and rinsing it, especially the sand contact areas, take a light whiff of the rocks as cleaning just to make sure they smell normal, rot will be obvious, these would be detritus pockets. The live rock is rinsed with clean water and kept wet, nothing dies from this. Things die when rotting detritus leaks ammonia around them, detritus is what's in your bed it's ideal to remove it, just not common. Avoidance and storage rules the day, it's literal hard work to keep big tanks clean as we like to bioload them.
So the rocks are detailed, cleaned, sniffed and in a water holding container of their own in case you didn't clean as thoroughly as mentioned
Now we are down to the goal: 8 yr sandbed
It's procedure is by far the most complicated of them all:
Rip it out throw away.
Replace with all new wet pack live sand, caribsea insta cycle, that was blast rinsed savagely before putting it in your tank. The working theme is no detritus, no silt, put your whole new tank back together with the water you chose. Acclimate fish and corals back. I wouldn't need to ammonia test in this event because I wouldn't transfer detritus, but for anyone that wants to make decisions off ammonia testing you must use salifert not API
The result of this lengthy procedure: no detritus reef in about 8 hours of work or less with help~
The reason nothing dies or recycles: detritus was considered, sorted in storage, isolated, and removed not in the presence of things that don't like to be burned with raw ammonia.
Bacteria never died even if you left your rocks on the porch for two hours, but you kept the live rock wet/submerged during the process except while rinsing in clean salt water etc. We retained all needed bacteria in the cleaned live rock and new rinsed sand, rinsing isn't antibacterial for adhered bacteria, what survives rinsing is plenty. The bac lost in the full water change were incidental; like the ones we rinsed away with detritus
If this deliberate action is too much work then a never caught up cautious cleaning risk of doom is a fine alt
as well as ATS, carbon dosing, coil denitrators and all the band aids we employ for bioloading and or waste storage... Large tanks are hard work acknowledged ~ this is why I must stay with gallon reefs
Not everyone wants to rip clean they want lots of time between big jobs, but 8 yrs is time for work. When I rip cleaned my own reef after nine years untouched 6 inch dsb, that's in a thread and this is the procedure I used so that I could save all life and start with the culled down version for next decade
We have threads on each of these procedures being done in other large tanks having to move homes, rip clean etc
