@LBSleonor to be honest from my perspective this model is not very good if you have such a sensitive and small target. What you can do is work on the data preparation and hyper parameter tuning to get somewhat better results and also try H2O.ai AutoML to cover several modeling approaches.
You can find an overview of models here. And maybe use AUCPR as the target statistics
More also on Data preparation with vtreat here:
Specifically about unbalanced datasets you can also check these two threads from the forum: