I agree with Kellerica. Just do more localized maps that interest you and if you want to stitch together a world maps later, do that. Consider the high likelihood that what you do now may not interest you as much in the future, and then approaching them will feel far more laborious and very constricting. Leaving the world open allows you to keep putting maps that you enjoy doing at the time you are doing them. Besides which you learn more as you do more, and that also has an effect.

That said for what you want to do, forget about zooming or even taking details from the main map, if you want a localized map draw that localized map entirely at the scale you want it. Consider the "zoom i" as more of a "block in/outline" and work from there.