Baal Teshuva: What Comes First
Rav Moshe Shternbuch: Shabbos Or Kashrus, Where Does A Baal Tshuva Start?
Rav Moshe Shternbuch says (1:350) that a Baal Tshuva is like a small child. He needs to start slowly and learn first how to lie down, than sit, and then walk. During this process the Baal Tshuva is still not complete and the person guiding them will need to choose wisely what they are ready to accept. It should not depend on the stringency of the aveira but rather on the ability of the Baal Tshuva.
Rav Shternbuch says that it is easier to start with Kashrus although it is only a Lav rather than Shabbos whose violators are Chayav Misa, capital punishment. The reason is, that finding alternative kosher food is relatively easy whereas keeping Shabbos is not. He cautions that burdening a Baal Tshuva with more than they can handle at each stage, is counterproductive and you may lose them altogether if they buckle under the load.
The main thing, he says, is to strengthen their Emuna that Hashem created the world and that there is reward and punishment. You need to ignite the spark and move forward judiciously.
Integrity over profits.