I do the same, except that I don't succeed in getting gold and I spend a lot of time everyday trying to do that. If I win 4M/month with TT's it is the max.My favorite way to earn credits is to do my best at the Online Time Trials, since there's a new one every week. I've entered most of them, and I've either gotten silver or gold - never bronze. Plus, it's a nice way to complete my daily workout.
I win much more credits if I grind Sardegna (and it gives me the DW too). In fact more like 4M/week at least.