OK, few thoughts from what you've said...
Yeah, just like farriers, there are 'barefoot trimmers' and 'barefoot trimmers' - some who are fanatics, some with little good study or experience, some with... interesting theories. Or otherwise. Just don't 'tar them the same' any more than you should judge all conventional farriers the same because of similar experiences with a couple.
I don't know what PNW rain means, but constant wet footing is not good, and yes, abscesses, particularly sub solar, as the sole is softened, are not uncommon in that situation. Is there any way you can keep him on dry footing at that time of year, at least part time? I hear you that you don't want to lock him up tho... that's not ideal, but maybe if he were locked up just over night on drying footing, if he had a mate & bigger area than just a stable box... Also, soaking feet in a saturated saline solution, or packing with salt & wrapping will help them dry out, by osmosis.
That it's just his hind feet indicate that there are likely mechanical probs going on too. Likewise for the 'leaky gut' hypothesis. It is absolutely a thing that gut probs, including 'leaky gut' from ulcers or such, which allow toxins to 'leak' into the blood, can cause laminitis. So it's possible that is part
of the prob. As systemic issues effect all feet equally, it could be his feet are slightly
compromised from gut probs, but there's still something else going on to cause the hinds to be so much worse. And again, 'hollow sound' indicates there is stretching/separation, so I'd say mechanical issues are the major prob.
Sounds like diet is good. Depending on what vits/mins he's getting from that will effect what he may need from supplementing, and if it's a generic type supp, it's possible it is not the best for him. Too much iron, too little Mg, Zinc or such. So it is best to do a diet analysis before working out what supps may be appropriate. If he's 'very prone' to thrush & dermatitis, IMO there's likely a nutritional factor there. *Some studies, by a rehab specialist in a wet environment(Tasmania) have shown that feeding MSM helps reduce probs such as seedy toe & thrush, so it's also worth looking into stuff like that. Again though, even if nutrition is a bit of an issue, it's obviously only a part, if it's only his hind feet effected so badly.
If you keep finding 'abscess tracks' each trim(where abouts? Under the sole or in the wall?) it sounds like there is a seedy toe/WLD issue likely - whether that's secondary to the abscesses or not. This really needs to be dealt with assertively, as it can be insidious & cause major damage if left. Where did the farrier dig the abscess from?
Sounds like he isn't all that active, if he lives in a dirt paddock & is only lightly worked, so more exercise will be good for his general health, hooves included. Is the horse shod in conventional rims? If so, unfortunately, especially as there are(educated assumption) mechanical probs, this will exacerbate those issues. This is one reason I wouldn't advise conventional rim shoes. They are very often a great palliative though, allowing the horse not to be in pain(or as much). He may well need protection & mechanical support that you can't provide well enough with boots, and as mentioned, there are other fixed alternatives to steel peripheral rims.
And again, esp as it sounds like it is due(mostly at least) to mechanical probs, if you'd like to post pics, may be able to give you more specific info there.