I think the most sensible way to draft and discuss an RFC would be with a tool like Jira/Trello/GitHub Projects. There can be multiple threads, sorted into columns like "questions", "concerns", "alternatives", etc.
This makes the discussion better structured, so RFCs with hundred of comments remain manageable, and provides a nice overview. If someone has a concern, they can add it to the "concerns" column. If the concern has already been raised before, they can upvote it.
The RFC document can be short and informal in the beginning, but should be revised and extended over time. This should be collaborative (so more than one person can have write access). There's no need for a separate "exploration phase" or a Pre-RFC.
I think that the whole community should be involved from the very beginning, so everyone can provide early feedback. Once the RFC document is complete, all the feedback has been addressed and hopefully community consensus has been reached, the RFC is done. Then it's up to the lang team to decide whether the motivation is strong enough to outweigh the drawbacks.