I've had issues with fungus gnats eating the roots of cactus (Trichocereus)seedlings.
While aware that essential oils commonly repel or even harm insects, I am also aware that they can cause harm to plants in some cases.

I have killed hundreds of seedlings in decades past through undiluted application of essential oils, however being frustrated with the gnats I considered that most plants conserve their basic botanical and biological properties and that essential oils are found at low concentrations in plant species where they often serve to protect the plants from insects and yet do not harm the plants.

In reviews of the use of essential oils I found various claims of them being used to prevent or treat insect pest issues without harming plants, provided they were at a low concentration.

So I began experimenting with the addition of cinnamon oil, from Cinnamomum cassia aka cassia oil, to the fertilizer solutions I was using for the seedlings.

I favor the frequent application of dilute fertilizer solutions on seedlings once they have germinated. I typically employ a 20:20:20 complete macro/micro synthetic fertilizer at a rate of about 1/4-1/2 teaspoon per gallon and use this concentration at every watering, using a pump sprayer to apply. I have the seedlings under artificial light and employ a fan to promote air movement.

I have employed 5-10 drops* of cassia oil per gallon and shake vigorously before spraying, I tested the oil at this concentration on established cuttings and noted no visual damage over a period of days and so then began using it on all of the seedlings. After about a month of use the number of gnats as well as their larva♥ has decreased significantly. I have let the media dry between applications. The seedlings have become increasingly vigorous and I transplanted several noting new and healthy root growth and a lack of fungus gnat larva.

I also applied this in a closed bag environment that had numerous little springtails and predatory mites. The population of springtails decreased but rebounded well, however the predatory mites while still present were affected more adversely.

While this is not conclusive the indication is that cassia oil in dilute application is not noticeably harmful to Trichocereus seedlings but does appear to control populations of fungus gnats. I am not aware of the actual mechanism of control at this time however it may be killing eggs as that for a time adults were present in higher numbers and were frequenting the surface of the media, however their numbers gradually decreased and new larva have not been observed when looked for.

* 10-12 drops of the cassia oil is about 1/2 milliliter in volume.

♥ A useful way to observe larva is to place slices of raw potato on the surface of the medium and life them up to examine them after several hours.

I've had fungus gnats problem with MJ grow, also tried various fruit and herbal oils without any satisfactory results. They are pretty persistent little buggers and even survived ACTELLIC pesticide application, though numbers had dwindled.

In the end - adjusting watering habits finally did it for me, and also physically separating plants to different rooms, so that you have multiple small areas to fight them, rather than one big happy bug playground

I made sure that at least top 2 cm of soil is dry before watering, so water less frequently but bigger volume of liquid helps to make their life considerably harder.

I've never grown cacti though.

I've noticed dead larva.

Cinnamon oil appears to be rather effective.

I let the soil dry in-between watering, it hasn't been effective as a control.
At the moment I am unable to use my normal means of control, 98% clarified hydrophobic extract of neem oil. It is food grade and works 100%.

I've been growing various plants for about 25 years.
Cacti for 20.

In this case I am just testing the effects of cinnamon oil and I am highly pleased.
Not all essential oils are equal.

ResearchGate has this

Haddi, Khalid & Faroni, Lêda & Oliveira, Eugenio. (2017). Cinnamon Oil. 118-150.