im def on the non-planted side of the debate. in fact im a bare-bottom tank lover! thats what i would recommend at this stage because u r new to discus and they r the easiest to maintain.
1) remove fish to bucket with heater
2) keep filter material wet
3) remove all water and gravel, wipe down tank, refill with water of same temp and dechlorinate
4) add filter back to tank
5) add fish back to tank
if u really want to keep the plants, i'd recommend to put them in little pots. u can always add sand later once ur little guys have some time to grow. a lot of ppl paint the undersides of their tanks a light colour if they go with BB.
light - 16 hrs a day is prob gonna lead to algae issues. i only run mine 6-8hrs (i have no plants, but my plants did well with 8hrs of light when i did have them)
nitrate - i suspect u need to retest this, all tanks have some amount of nitrate if the cycle is complete (is the cycle complete?) if u r using the api liquid test u may need to really bang and shake the liquid bottles to get a proper reading.
ammonia - does ur tap water have ammonia straight from the faucet? there should be no ammonia if ur tank is cycled. its possibl uneaten food ia rotting and maybe ur filter cant handle the bioload and u do need more filtration... OR ur tank is still cycling... removing the gravel should help with cleanliness, but removing gravel can also remove some of your beneficial bacteria which may be living on the gravel. if u remove the gravel u may need to increase water changes while the removed BB grows back elsewhere (eg in the filter). make sure u test ur water regularily for a while if u remove the gravel.
food - more variety is great. my discus love freeze dried black worms the best. be aware live food does carry risks, tho some ppl swear by it. i cant feed frozen bloodworm anymore myself because im allergic to it.
heater - placement is ok. i like to lay mine across the bottom so i can wc very low without exposing it, plus i feel there is a more even heat distribution but its just my preference.
and welcome to the forum! excited to see pics of ur tank as it evolves