I have written complaints about several well-known casino sites, but I have not received a response yet. What else can I do?
If they are licensed then try to reach out to the license provider. You can ask legal aid. But considering the amount you are claiming, going though all these additional work really not worth it. Next time avoid the casino.
I would suggest the same like the user above me. You should be putting some evidence of your claim otherwise you are asking others to believe your words.