What's the difference between a submissive and a slave?

Duration and scope, mostly. A submissive hands over control within agreed limits, often within particular scenes or areas of life. Someone in a Master/slave dynamic has handed over a structure that continues when nothing is happening.

The word is heavy and the community is aware of it. What it describes is a negotiated agreement between consenting adults, usually written down, and always revocable whatever the language inside it says.

Neither is further along than the other. A lot of very experienced people are submissive and have no interest in anything resembling ownership.