We had a 6" DSB sugar sandbed for 4 years.
Currently bare bottom.
I was new to salt water and in spite of a lot of reading totally clueless.
The DSB can act like a kitchen sponge. Doing an amazing job of absorbing the bad stuff.
I kept sps, lps and a ridiculous fish load.
The "kitchen sponge" absorbed all my horrible mistakes.
Until the "kitchen sponge" gets saturated.
And then it's similar to dragging the sopping wet sponge from the spill on the kitchen table back to the sink.
Then you are done and your tank dies.
You would not believe the amount of filth that can fit between those almost microscopic sand grains.
I wish I did a video taking the sand and filth out of the 75 gallon tank.
I would recommend that you go slow and clean 1/6th or less at a time and clean that 1/6th thoroughly.
Then a few weeks later do the next section.
If I had done this properly I have been told the 6" DSB would have lasted at least a decade.
Year 3
Then the end of year 4 before the complete redo.