Any updates or luck getting a motion video?
I would try to "rule out to rule in" because symptoms can overlap or look similar
Dehydration - it should have another water dish right by it, moistened substrate as long as you have the ventilation to support it
Build up of Nitrogen/ammonia, oxygen displacement. Here is an example of how I place my ventilation holes to achieve a lower moist layer, a dry top layer and to give it a chance to dry out in between waterings if I need it to.
I try to strategically plant moss into a corner behind the water dish (front right corner in the picture) so I can over water there and allow the soil to wick/distribute the moisture. That's what soil and moss do, suck the moisture out of the water dish. It can easily lead to swampy conditions. This could easily suffocate my tarantula without all the ventilation at the substrate level. And it would look perfectly fine up until you happen to catch signs and symptoms or it starts to smell.
Trauma induced hypovolemia
This could be a freak accident, could be a tiny perforation that occured prior to the last molt, that didn't fully repair, or it could have happened while molting. So anything looking wet, on the T or the substrate. Check the old molt with some back lighting for holes, tears, dark spots.
With a keen eye you need to rule out potential fluid loss then can probably move onto the next bad scenario - exposure - and definitely before you attempt moving it (more on that below)
But first a PSA
For anybody reading WATCH THOSE BRISTLES AND THE METAL CLASP AROUND THEM (sharp). You always need to be mindful of the direction and proximity to the abdomen. They can poke holes in a freshly molted T. Aim for the legs only to be safe, be prepared for any reaction. Get yourself some plastic only brushes (the really cheap ones that are like $4 for 20 at Walmart, like kids ones. No metal clasps to worry about ever. Just need to practice dropping it if your T is hungry
For this particular T, I'd take advantage of its inability to move and immediately place a cup over it, so in the offchance it can magically bolt, you don't have to worry about it hurting itself. Then I would take a lid and slide it under substrate just enough to prevent contact with the T but to also give a little support to the T in case you have to lift it out on the lid (and it won't walk into the cup)...does that make sense? Like a little dirt pillow? Having the cup over it will also reduce stimuli like air movement, and since your T is already feeling vulnerable we don't want to stress it out even more.
Exposure
In theory if there is nothing wrong with the rest of your collection, you can probably rule out the room and hone in on the enclosure. A total rehouse, also sanitize the enclosure with a safe surfactant like unscented biodegradable dish soap and then wipe down with alcohol. Substrate like reptisoil or something marketed for pets from a pet store, not some generic coco from Amazon if you insist on coco
DKS
You already read the temperature trial posts. I'm not saying to induce a fever. Rule out to rule in. If it were any of the aforementioned problems, DKS "treatment" will kill your T deader than it would've died on its own. What I suggest is to induce Spring, gradually bump your T a few degrees by keeping it on the top shelf, as warmer temps increase metabolism and would hopefully encourage an appetite. It's why we see so much more growth and activity in the warmer months right?
I would try to "rule out to rule in" because symptoms can overlap or look similar
Dehydration - it should have another water dish right by it, moistened substrate as long as you have the ventilation to support it
Build up of Nitrogen/ammonia, oxygen displacement. Here is an example of how I place my ventilation holes to achieve a lower moist layer, a dry top layer and to give it a chance to dry out in between waterings if I need it to.
I try to strategically plant moss into a corner behind the water dish (front right corner in the picture) so I can over water there and allow the soil to wick/distribute the moisture. That's what soil and moss do, suck the moisture out of the water dish. It can easily lead to swampy conditions. This could easily suffocate my tarantula without all the ventilation at the substrate level. And it would look perfectly fine up until you happen to catch signs and symptoms or it starts to smell.
Trauma induced hypovolemia
This could be a freak accident, could be a tiny perforation that occured prior to the last molt, that didn't fully repair, or it could have happened while molting. So anything looking wet, on the T or the substrate. Check the old molt with some back lighting for holes, tears, dark spots.
With a keen eye you need to rule out potential fluid loss then can probably move onto the next bad scenario - exposure - and definitely before you attempt moving it (more on that below)
But first a PSA
For anybody reading WATCH THOSE BRISTLES AND THE METAL CLASP AROUND THEM (sharp). You always need to be mindful of the direction and proximity to the abdomen. They can poke holes in a freshly molted T. Aim for the legs only to be safe, be prepared for any reaction. Get yourself some plastic only brushes (the really cheap ones that are like $4 for 20 at Walmart, like kids ones. No metal clasps to worry about ever. Just need to practice dropping it if your T is hungry
For this particular T, I'd take advantage of its inability to move and immediately place a cup over it, so in the offchance it can magically bolt, you don't have to worry about it hurting itself. Then I would take a lid and slide it under substrate just enough to prevent contact with the T but to also give a little support to the T in case you have to lift it out on the lid (and it won't walk into the cup)...does that make sense? Like a little dirt pillow? Having the cup over it will also reduce stimuli like air movement, and since your T is already feeling vulnerable we don't want to stress it out even more.
Exposure
In theory if there is nothing wrong with the rest of your collection, you can probably rule out the room and hone in on the enclosure. A total rehouse, also sanitize the enclosure with a safe surfactant like unscented biodegradable dish soap and then wipe down with alcohol. Substrate like reptisoil or something marketed for pets from a pet store, not some generic coco from Amazon if you insist on coco
DKS
You already read the temperature trial posts. I'm not saying to induce a fever. Rule out to rule in. If it were any of the aforementioned problems, DKS "treatment" will kill your T deader than it would've died on its own. What I suggest is to induce Spring, gradually bump your T a few degrees by keeping it on the top shelf, as warmer temps increase metabolism and would hopefully encourage an appetite. It's why we see so much more growth and activity in the warmer months right?