Hey there @oofmadone,
I tagged @flannelist for input on this question. They have some really good insights for Steam Deck issues and even wrote our troubleshooting pages for this device. I'm willing to bet that it is a partitioning issue with the SSD since you should be able to upgrade your SSD to 2TB without issue. I would also maybe check out the [guide|148989|Steam Deck SSD Replacement Guide] and see if a step was missed during the upgrade process.