Merchants in a TF? Doesn't sound like any task force I ever saw! I think those "task forces" you're intercepting are actually convoys.
I've found a few juicy targets in GWX task forces. Early in the war, the British TFs I encountered tended to have Auxilliary Cruisers in them (~13K each) and, rarely, a battleship or two. Mid-war I've found aircraft carriers and more battleships.
Trust me the big military ships are there to be found.
