I think you would only need half the usual credits due to the summer eupgrade promotion, so you should still have some left over after that trip.

I don't find J on the CRA to be particularly nice as compared to the E190 or Airbus fleet. Plus, Y on the CRA isn't all that bad, and if you can score row 12 you get a ton of legroom.

However, if the credits would just go to waste, then why not? I've got a roundtrip on the CRA coming up and plan to use my credits if there is space available. I'm not likely to be doing any long hauls this year (unusually for me as I normally do 2-3 TATL per year) so have no need of the credits there, and most of my US travel is on UA, so I don't have a problem with "wasting" credits on a CRA.