Cabagge can be tough on stomach and digestion, can make the gassy. I would dich it. I would rather start with herbs. Small amount. Ant then har leafy things like fresh dandelion, arugula, mache, leaf of beetroot, swiss chard, spinach, green leaves of carrot... Foods with high oxalic accid not be feed a lot, nor daily (parsley is one of them).
Thank you very much for the recommendations! I didn't know that about oxalic acid, I will definitely bear that in mind.