y not said: “As far as I can see, all that one can do is to constantly re-enforce one’s good gathi (by staying away from akusala, observing the precepts,mindfulness, doing Metta Bhavana, Pattidana, and dana of any type), thus re-enforcing those kamma with good vipaka.”
Yes. Only thing we can do is to stay on the Noble Path. The Buddha said that the only thing a farmer can do is to take care of the field by providing water, nutrients, etc. A good harvest will result. No point is worrying about whether one will have a good harvest.