Although there is no string at all for the bonus, but I dont think players will try to withdraw it without playing because KYC is needed in order to open the banking/wallet menu to deposit/withdraw. As OP said as well, by letting people to withdraw it straight away, it will also give them a feedback for their withdrawal system. The bonus is also limited to 50 users only, so even if all people try to withdraw it straight away, they will only spend $250 but I'm pretty sure no one will do KYC to withdraw $5 so the most possible case is that people will try to play it first trying to make more with the $5 in order to make it worth for KYC.
KYC isn't mandatory to withdraw your coins. Correct me OP if I'm wrong.
Level-1 KYC is there just to check who is sending money. It's better to ask for some basic personal information than to seize your find later. Besides, as OP said, you can send the level 1 KYC information anytime if you want to go further and submit full KYC information. So, it should not be a problem.
OP please share a clear picture about KYC as people seem very much confused.