Interesting use of "so much"

This is the OED's sense 39 c. under head-word so:

c. adj. An equal sum or amount of (something).

One of the examples given is :

1885 E. Lynn Linton Autobiogr. Christopher Kirkland I. 219 Even my languages..were merely so much literary furniture.

Sense 37d. is the equivalent, for so many.

I think this sense is only used in a dismissive way, implying that whatever is compared is of little worth.

You threw them away like so much trash means You threw them away as if they were trash.

like so much idiom

: like something that is
_{// The explanation sounded like so much nonsense.
// The house burned like so much paper.}

_{Source: Merriam Webster — like so much}

Your definition is correct and applicable here: “so much” = “this amount of”. The implication is that trash or ash is all that the item is, now. It suggests ignorance, or wilful dismissal, of any other value that the item may hold (or have held).

You threw them away like so much trash.

You threw them away like an amount of trash, with no consideration for their value.

The bedspread was so much ash.

The bedspread was reduced to merely this amount of ash, with no other worth.

In my experience (Australian English), this is an uncommon use of the phrase in modern speech, but it’s still understood well enough in writing.

You might also want to look up the definition and uses of the phrase "So much for ...", which is another idiomatic usage with some similarities.