You could use Other World Mapper (a lower cost, but not free program), which lets you very easily draw a fractal landmass and save styles for them. The reason I would suggest it for you is because it can import GIS data, which is what you're wondering about for how to quickly get a map of this world. That's right, there are techniques for that. Won't save you all the time you spend styling it after the fact, but a free program that will let you work with just GIS data is Qgis.

Wikipedia is where I would go for generic SVG files to tinker with, you can also pull those into Other World Mapper.

If you mean you're going to be doing more historical projects in other time periods and how do we find things to reference for that? https://www.oldmapsonline.org