I’ve dealt with root aphids on a larger scale before and I can add a few things to the conversation here.
First off none of the pictures you’ve posted so far look like root aphids. As others have noted, the mite creature you have captured is most likely a beneficial soil mite Hypoaspis Miles so nothing to worry about there aside from killing them with the Triazicide. (RIP).
Secondly, the fungus gnat larvae arent a major issue and their increased presence in the pot mostly lends credence to the theory that the poor plant is just overwatered. This is still the only plant in the greenhouse exhibiting these symptoms right? One thing about root aphids is that they are going to spread incredibly fast; mostly because they can produce a winged form when their population is high in one place in order to spread to another. See below:
These are going to look slightly like fungus gnats, but the main difference is a pair of “Cornicles” emerging from the back of the winged aphid, and any aphid really, which is one of the easiest identifiers. Cornicles look like little exhaust pipes coming out of their ass, best way i can descibe it.
You’re more likely to see the root aphids on the surface of the soil, as well as clustering around the stem itself and slightly up the stem stuck onto sticky bracts or any flower material thats lower on the plant.
Good preventative treatment for root aphids include beneficial soil predators such as Hypoaspis Miles (RIP) or Dalotia Coriaria; I prefer Coriaria since they are easy enough to rear on your own, if you would like to know how just drop me a DM. In addition to insect predators you should also make use of Beneficial Nematodes, I used a mix combination of Steinerma Feltia and Steinernema carpocapsae and they help prevent not only root aphids but also other soil pests from establishing a population.
When you have to go the chemical control route, please please please do not use Imidacloprin. I’ve seen a few comments recommend that but its a systemic insecticide and will unequivocally remain in the flower after harvest even if you clone off the plant and grow that out. That’s why you can’t use that shit in any situation where you get tested for pesticides.
When you need to hit them with the gas, go for an Azadirachtin product. I like to use Molt-X specifically for root aphids but also as a deterrent for other soil pests, and its fairly gentle on any beneficial populations you might have (RIP Hypoaspis).
Also, in my experience, Pyrethrin have not been that effective at nuking root aphid populations.
I know I kind of went on a ramble here so if I can clarify anything just ask.