Exasperating seedling growth

Hi.
Ive taken up garden recently.
I started sowing easy annuals (zinnias, marigolds, cosmos) and despite my best efforts, I quite do not know what has happened.
My seeds germinated quite well simulating green house (place them in sterile cups, good drainage, backyard soil then covered them). I place them in direct sunlight, weaning them gradually (summer's here are hot, about above 80s, Zone 11). Some of plants grew, then suddenly tipped over with tapered ends on ground surface which I figured was the damping off.

So with the other seedlings, I made sure they were in well aerated areas, watered evenly, waiting for the surface to dry out before watering again 1-2 days later. Also used (chamomile tea, and cinnamon powder on surface).

But the results have been...exasperating. Some have become spindly (despite abundance of direct sun), some have brown leaves or become discolored to yellow-white or mottled white) and some looked mummified (wilted brown dry).

I examined them. No aphids etc. Though in a few, I saw fine gossamers . Probably spider mites so I became vigilant about these - washing the leaves gently, separating the afflicted ones from the non afflicted. Etc.

Regardless, they seem to be turning brown and not thriving well.

I used balanced fertilizers 10-10-10 on them once a week, twice already in some; no fertilizers for others.

The results again are the same: stunted growth, spindly like vines, brown leaves or discolored ones, mottled white in others; while some have literally wilted brown dry.

See pics below. Please shed some light. I do not know what went wrong, what i did wrong and how i could salvage or improve next time.

Thanks

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