not sure on a substrate for nano tanks

[mmm55645]

I have a 10, 15 and 20 gallon tank, at least one of which I will be using as a saltwater tank.

My plan is to run the tank with ample LR and flow, very lightly stocked(FOWLR) without a PS(until I create a DIY one), and 10% water changes a week or every other week. I will probably only keep fish in the tank until I am sure I can keep parameters in check.

I have read contrasting views about what works best as a substrate.

Is it true that it is not worth trying to have a DSB in a small tank? I thought I heard something about there not being enough space for the DSB to be effective.

I was thinking of making a rock bottom with quick setting cement if a DSB would not make sense., or just having a small sand bed.I haven't read much about this though... I would like to learn more.

If I go with a small sand bed and the tank is FOWLR I'm not sure what I will do to keep the sand bed sifted. I'm not sure that I want a sifting fish if the tank will look like a sandstorm. How do you clean(or is there a need to clean) a sand bed when there are no organisms to do it? Do you siphon it out like cleaning gravel beds in FW tanks, or will the sand be too light?
You can't just let it alone or bad stuff will happen, right?

thanks for any feedback