I fought the battle you are fighting for years. I now I have a small area I am able to keep at about 55% humidity and 60-62 F. Plants hang whole for 3-4 days and remain fresh, smelly and sticky until I get them trimmed and in containers where I spend about a week sweating them down to 62% rh where I leave them for longer storage. I have never used an humidity packs.
I didn’t read through all 4 pages of this, so some of these tips might be redundant or maybe you are already doing most of it, but here are some things that might help.
I think the room you have should be fine, maybe not for 12 plants at once, if I caught that correctly? That is a pretty big harvest, but I think that space should be able to hang dry 5-6 plants.
Is the closet reasonable well sealed, or pretty drafty? My room is larger and pretty well sealed, about a 4ft x 5ft tiled bathroom. When I have 3-4 plants that are freshly chopped I barely need to run the humidifier since the plants are putting off so much moisture. Is the closet possible very drafty and all the humidity is getting sucked into an un-insulated bit of floor or wall? Sealing the closet up more will help if that is the case, and the space should be large enough to run a small humidifier on a low setting.
I would recommend abandoning that hygrometer you have and getting one of these for your curing containers:
They cost a bit more but are generally very accurate. This is the type of hygrometer you should use in a sealed container, not for getting whole room reading.
For the temperature and humidity in your drying room you should get something like this:
Amazon product ASIN B01ALO500E
A unit like this is designed to give you more accurate readings in a larger environment.
Hopefully some of this is helpful, you don’t sound like a newbie, so you are probably getting close to dialing it in. If you really aren’t getting any lasting terpenoids in your dried flowers I would also look at your harvest times to make sure you are not picking a little early. I have had some grows that look great but the terps are the last thing to fully develop and don’t really show up until just as my trichomes are starting to shift amber.
good luck!