Would it be fair to do something where you start off with the BSL approach where older code automatically becomes fully open source (like GPL or Apache), with the exceptions that the first N versions or N years of work will start off fully open, then the BSL stuff will kick in later?

I think this would be fair, and it has been done. Ghostscript had two branches, Aladdin Ghostscript and GNU Ghostscript, where GNU Ghostscript is approximately one year behind.