You can use need oil to spray them this will kill all of them.
Neem oil is organic and none toxyck for human. You can even use it in flower .
You can put lear from Dolominite over the soil and kill the spider mites. But Dolomite can change your ph. (do not use too much) and in dolomomite have magnesium .(it is good for magnesium deficient )
If you not over water and allowed top layer of soil to dry you will not have problems with fungus gnats.
If you make drought/dry your soil/pot for few days you can fix the problem with fungus gnats without to use anything .
To Prevent spider mites you need to have good hygiene in the garden/growbox. Not to bring new flowers or even to bring them you need to decontaminate before to put them in the garden/growbox .
Many time i buy fresh flowers in a pot from gardening shop and they was with spider mites and infect every other my plant. For that i start to decontaminate every plant that i bring from outside from my garden.
You can use ladybugs or other predators to proteckt your garnden.
Use nature to your advantage.