Here be minor spoilers...

I'm in the drowned nations, with TNO as a mage, Morte, and Dakkon. Everyone's inventory is full, and I can't bring myself to drop or sell anything because I fear I will need it later. What can/should I get rid of, and what do I definitely need to keep?

Consumables:
Bone charm
Clot charm
Cranium rat charm
Charm of infinite recall
Blood charm
Needle & thread
Knot charm
Cranium rat tail
Dirty rat charm
Heart charm
Charcoal charm
Bandages
Jar of enbalming fluid
Corpse fly charm

Equippables
Obsidian Earring
Uhir's Knife
Enchanted Battle Axe
Leg bone club
Iron Prybar
Severed Arm (Yours)
Punch daggers of Moorin
Scalpel
Club of nettles

Priest scrolls
Scroll of cure serious wounds
Scroll of heal
Scroll of cure moderate wounds
Scroll of cure critical wounds
Scroll of cure light wounds
Scroll of speak with dead

Usables:
Abyssal pipe
Negative token
Sadistic frame
Shamanic rod
Whispering flask
Clipped copper piece

Readables:
Bronze sphere
The unbroken circle of Zerthimon
Straham's diary
Penn's note

Other
Junk
Decanter of Endless Water
Rags
Soego's Skull
Skull
Intestines
Were-rat skull
Mortuary sanctum key
Cobblestone
Phineas' key

First, if you didn't already, pack as much as you can into chars' quickslots (weapons/consumables/usables).

Things to keep:
Uhir's Knife
Severed Arm (Yours)
Iron Prybar
Readables
Decanter of Endless Water
Soego's Skull
Intestines

You can throw away rags, other skulls and cobblestone.

These scrolls and consumables are non-critical, so you can keep the most useful ones and get rid of the rest.

If you find you are stockpiling consumables and not using them, you can afford to shed some.

The game is not going to suddenly get much harder, or anything. Peaks and valleys, yes, but nothing dramatic. And it's not like you are going to get stuck reloading the same saved game over and over to beat an encounter. There are perks to being immortal.