Is it possible to create a CSV Inventory Adjustment, where you set the new value of the selected items? Not using "Adjust quantity by" but directly setting the new value.

Value as in valuation amount, or quantity?

Quantity

In either scenario, it sounds like you want Inventory Adjustment Worksheet

Yes I believe so

Inventory Adjustment Worksheet is manual, right? I'm looking for a way to mass update the correct quantity of 12,000+ items.

"The Single Inventory Worksheet Import provides an alternative that you can use to create a new inventory worksheet in NetSuite. With this import, you can submit inventory adjustment data in a CSV file that can be built from a system-provided template.'

With an IAW you set the "New Qty" and "New Value", it will "override" the previous Qty and Value. You can import a CSV here: https://system.netsuite.com/app/setup/assistants/nsimport/simpleimport.nl?rectype=INVENTORYWORKSHEET&whence=

there you go !

It should go without saying (but I will say it anyway!), make sure you do a test import with a few SKU's before importing your whole catalogue!

Thanks mates!!!