Compute difference between Subtotals

Right, again, no sample data so can't be specific. Hint.

But, in general, you want to do this. Take a look at your visual and see how it is aggregated. Let's say it is by "Thing". You want to create a virtual table using a VAR that emulates this and your columns. Then use SUMX and basic math. Something like this:

Subtotal Difference =
  VAR __Table = 
    SUMMARIZE(
      'Table',
      [Thing],
      "2019",SUM([2019),
      "2020",SUM([2020)
    )
RETURN
  SUMX(__Table,[2019]) - SUMX(__Table,[2020])