How do I express a time point which is a decade ago, counting from another time point mentioned in a passage?

Solution 1:

Using ago is ambiguous at best, and misleading at worst. (Because ago is naturally assumed to be relative to now, not the other date.)

You are free to give a specific date.

Or you can use some other words instead of ago:

. . . was not used in research until 2000, despite being invented a decade prior.
. . . was not used in research until 2000, despite being invented a decade before then.
. . . was not used in research until 2000, despite being invented in the previous decade.

Solution 2:

"xxx was almost impossible to use in researches until 2000s despite being invented a decade earlier."

Collins:

earlier

2. adverb [ADVERB with verb]
   Earlier is used to refer to a point or period in time before the present or before the one you are talking about.

COBUILD Advanced English Dictionary. Copyright © HarperCollins Publishers

Usage Example from Wikipedia:

Columbia was destroyed at about 09:00 EST on February 1, 2003 while re-entering the atmosphere after a 16-day scientific mission. The Columbia Accident Investigation Board determined that a hole was punctured in the leading edge on one of Columbia's wings, which was made of a carbon composite. The hole had formed when a piece of insulating foam from the external fuel tank peeled off during the launch 16 days earlier and struck the shuttle's left wing.

Here it is clear that 16 days is with respect to Feb 1, 2003 and not now.

Solution 3:

The correct adverb would be either beforehand or @alwayslearning's suggestion of earlier, but the verb tense is also affected.

xxx remained unused in research until the 2000s despite having been invented a decade beforehand

The verb form here reinforces that the invention is already in the past, at the point being discussed.

But it's probably clearer if you stick to chronological order:

Despite being invented in the 1990s, xxx remained unused in research for another decade.