For a start, there is only a certain amount of time that you are allowed to work, which translates to approximately 120% maximum possible employment. Furthermore, as far as I remember you are not allowed to take a significant side work if your main employer doesn’t agree.
On the other hand, once your contract with your previous employer is finished, you have close to 0 obligations towards them. You can tell them that this is their problem now and they should have increased your salary before to prevent you from leaving.
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