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.