Thanks, I got it working this way!
Would love to share, but I guess it’s not practicable to replicate since I’m not a coder, but a terrible hacker. 
I let Discourse fire a webhook for a user event, then use Zapier to filter for a user creation event
Then I trigger another webhook, this time in Zapier to get the Bot saas flowxo.com going which asks a Google spreadsheet for the messages to send to the user and stores the progress in another spreadsheet. Your hint helped to get rid off Zapier for the rest of the interview process.
In the end Flowxo puts all questions and answers in one string, posts it in the thread and turns the post into a wiki, so that the user can make final changes. If he then answers with “ready”, the wiki post gets replicated in a public thread together with a link to the profile of the new user. He gets a nice badge and that’s it.
We will test this next week with quite a few users who are on our “waiting list”. I will report back, if the conversion is good. Maybe it’s worth becoming a plugin…