Sorry to resurrect an older thread - but it needs to be known, as another poster said, that these are NOT RA. You can use the first two pics to discern 8 legs. The first pic shows the rear two sets of legs, the second pic shows the front two sets of legs. Also, the mouthparts are very mite-ish. They do resemble H. Miles, but from what I see, these mites are much smaller (by using the root for scale, and referring to them as a "speck"). H. Miles is a relatively large mite (for a mite) and they won't primarily be seen on the roots. Moreso just kind of roaming aimlessly through the medium - and from what I understand, primarily like to hang out in the top 1" of the medium.
I am 100% certain that these are some sort of mite - which is scary, if they are indeed the culprit - which the OP seems to be quite sure of that. But, it's scary because it is relatively commonly said that mites in the rootzone are no biggie... usually beneficial in some way. I have seen various growers report root-damage, leading to whole plant problems with some type of soil/rootzone mite present, while ruling out RA or another pest. And I'm talking battles, not easily otherwise written-off problems. I remember one guy on IC that had these "white speck" mites that he was convinced were keeping him from a successful harvest - for years. Tried every product under the sun, tried breaking down, closing shop for 6 months, sterilizing area and equip, only for the hell to reoccur. I don't know what his outcome was, he just stopped posting. I could find the thread...
Personally, I have H. Miles, springtails, and these little WHITE SPECK mite-like creatures - but, mine are definitely different than the OP's. And surely have 8 legs. I, too, have had root problems... resulting in "phantom" lockouts/deficiencies. Only difference, is the problems are not constant, and I still have great quality and pretty successful harvests. But as a passionate grower with a decade of experience, whom provides for several patients, can tell something is "off." The plants seem to be most affected when young, then flourish once of larger size in larger pots while in veg. Then, sometimes in mid-late flower, it seems as though some plants need extra care, and the entire garden seems more light sensitive than usual. This has been through a few gardens... with some in organic soil (best results), some using liquid nutes, which were the worst results - like I couldn't feed no matter how low the dose without seeing weird lockouts until I continued with fresh water. Tried different aeration ratios, different water source, different base soils, added more air circulation... all kinds of little tweeks here and there.... I hate to say it but I've just learned to deal with it after using a few products. Basically it's just like the plants need extra care, and growing seemed much easier in the past - no matter how much I've re-simplified. I was convinced I had RA, and searched high and low for any form of aphid, or other pest to blame... thrips, broads... etc. Bottom line is, I know my rootzone is affected negatively, by something. I've never seen any pest directly on my roots like in the OP... but there's no question my roots aren't healthy as they once were.
I, and from others' advice, have speculated the arrival of soil mites due to some sort of bad bacteria/fungus/pathogen... being as how unlikely it supposedly is that any mites are directly feeding off the roots - as in, they're feeding on whatever it is affecting the roots.
I don't know, but it's quite interesting to say the least.
Sorry for the long post amd resurrection, I just believe there's something more sinister invading rootzones than just RA's and the well-known. Oh, and did I say the pics in the OP are definitely mites? ;)