I personally prefer KYC at the beginning rather than having to KYC after wanting to withdraw big wins, it will be more complicated.
but i do KYC early usually at a reputable casino here and Rollbit one of the good reputable casino. so you don't hesitate to do KYC at the beginning and are more comfortable when you are going to withdraw large funds without asking for KYC again.
It's rare to find players who want to do KYC at the beginning, especially when registration is not directed to be required to do so and also sometimes casinos include KYC not as a requirement except in certain circumstances such as large deposits and large withdrawals or when there are suspicions that arise, I prefer not to do KYC if it is possible but if it is a must then there is no other choice but to comply.
I think asking it upfront is a lot easier, because it sets the tone for everything else later on. If you do it upfront then it means you can gamble with ease without having any worries about the future potential large deposit or withdrawal or any suspicious activity or anything like that because you are already known and you already gave all your information as well.
Whereas if it's not asked upfront but later on when the casino wants to, that means they can let you gamble and lose all you want, but when the time comes and you get lucky a bit, they may keep it and first ask you to do KYC and all the other things and just postpone the payment under guise of "we are checking the information" for over a week.
I think it depends on the perspective of the gamblers. I prefer having it later on. Not everyone is receiving a KYC request especially if they just only wants to play gambling games on a moderation like depositing small and withdrawing small amount of profits. I think most who are getting a KYC request from a casino are the one who are taking out big amounts of profits and those who wins because of promotions. As long as you don't have not doing a thing against their ToS, wins big or took a promotion, I don't think they will request a KYC from you. I'm sure there are people like me who likes doing small bets and just like enjoying a small time gambling. I believe those people don't get asked KYC too much and submitting it first before playing on a casino will surely be a hassle to them since we just want a leisure and not a hassle of submitting KYC.