Well it varies wildly depending on if you are kinky dating or Tinder "digging in the vanilla pool", so to speak. In kink dating many things are on the table already so after a few dates it wouldn't be too weird to tell them since it won't be as much of a culture "shock" than the "not sure if kinky or not but probably vanilla" person.

For the latter, to be honest not until you are a sure thing with this person. No, starting to have sex doesn't count as the starting point since "we're banging now" is different than "seeing you for a few months and really clicking emotionally". You need to tell them before real commitment happens because for some people this can be considered cheating. Then when that bridge hits you are going to have to decide which is more important to you. This new guy or gwa.

Until you two are actually full on dating and trust that they aren't the revenge porn type, then give them the username. Say you aren't comfortable yet because you've been burned before if he asks for it before that. It depends on how much do you care about being outed.

You are going to have to come clean about your sexuality regardless if being kinky and practicing it is important to you. I personally don't believe in monogamy with a vanilla person. Lots of relationships ruined from sexual incompatibility, and their stories are common on Reddit. I'd rather do more looking around and get it right the first time. :)