Should I invest more time on old problems or new problems?
My Bayesian advice is to go with the lower prestige journals and move on. Quantity has a quality of its own.  Plus you learn more as you go on.  Don't get snarled up in reviewer, editor debates.  Keep publishing, keep producing.  The quality will come.
BUT also, I would work on your clarity in written communication (using this question as evidence).  That often holds back a borderline paper or allows one to move through.