I seems everything in bitcoin exchanges as something that's not secret, and for we not be traced before using exchanges to convert bitcoin to fiat currency, I think we have to use mixer first and after using mixer we can transfer to exchanges, that will be less trace
I don't get your point. Can you buy bitcoin from a mixer service with fiat? However, it is not good for you to use a mixer service and send your bitcoin directly to a centralized exchange because your account would be freeze. You are also not to send directly from an exchange to a mixer.
When using a mixer, you are to send it first to your noncustodial wallet before sending it to your exchange account. It's the same when you want to use a mixer and your coins is in the exchange, you are to send first to your noncustodial wallet before sending it to a mixer.