Unfortunately, we still don't have full blending option. We can merge widgets of different marketing channels, but they just present their values one beside other. They don't avg them :)

Google Sheets integration can help with those specific metrics you want to blend (I presume you are doing that for just a few metrics, not for all).
Connect those few APIs in Sheets, pull Sheets in REportz dash and it will work.

"Solution. Not the best solution. Solution."
(my dear colleague head of development thought me that:))