Why did you send your coins directly from casino to mixer? you can use non custodial wallet first.
Based on my experience, the casino neither block my withdrawal or account, as long as you've submit your KYC. Usually the casino will flag your account if you receive/send the coins related to mixing service.
2. use a trusted mixer that has good aml score and clean coins
My question, how to know the mixer has a good AML score and clean coins?
You need to check first. Mix your coins and use a AML core checker or read reviews about the coins they send.
https://bitlist.co/mixers also shows the AML risk and reviews for a few mixers
Why take the risk? If you're going to use a mixer anyways, why not use a regular wallet to send funds to and from the gambling site, then use a mixer to wash the coins later? Seems like a silly predicament to even consider putting yourself in honestly.
Some gambling sites can also deny coins I think because they believe the coins are dirty and you want to use the casino just to clean the bitcoins