You should buy the DI portion of the unit. You will need it for 0 TDS.

No clams with puffers or triggers. If it has teeth, or, lol a tooth, it will chomp down on corals clams and shrimp most often times. You will need a large skimmer to skim out all of the dooooooo.

don't worry about a powerhead falling. I have a good amount of sand in my tank and I have never had a problem with a powerhead falling and killing everything. Just alot of dust on corals and alot of slime.

Your rocks should touch the bottom glass, with that DSB in there, it could get unstable. A fish could dig a nice home underneath it and cause some wreckage.

Have you ever used copper in that tank? If so you will need to re-do the silicon. Copper will kill, well everything but fish, basically.

Don't overthink it, but you will need live rock.
You will need a skimmer, and a big one.
It would be easier for you to start with small fish until you get the proper setup finished before you just kill alot of fish. You need to be able to export nutrients, no fuge, no skimmer, no LR, and only sand + heavy bioload = disaster waiting to happen.

You can't circumvent nature that much