A coastline becomes more convincing when it is built in layers. Start with the large silhouette, then work inward toward bays, inlets, islands, and smaller interruptions instead of drawing every wiggle at once.
On an aged surface, restraint matters. The parchment already contributes variation and visual history, so the coastline does not need to carry every bit of texture by itself.
Mapmaker Sheets are useful here because the page can act as both working surface and presentation layer. You can test geography, labels, symbols, and notes while keeping the whole draft inside the same material language.