CC3+ is certainly capable of doing world maps, but it isn't really intended to make a world map with such a high amount of detail that you can basically zoom into the local level on the same map.

I am not sure about the exact limitations, but I am not surprised you might be encountering some. A quick calculation shows you are asking for something like 11 million hexes here.

I would strongly recommend you do not attempt to make a map in that high a detail level. Not because of program limitations, but because of mapping fatigue. Making a world map where you can zoom into anything sounds like a great idea on paper, but in reality, the effort of mapping the entire world in such detail leads to mapping fatigue and partial maps one never complete.

Mapping world maps in CC3+ is much better done like they are done in the community atlas, where where have a nice world map, but without too much details, and then the continent have been developed as new maps of higher detail. This makes it more easy to get the map done, and then work on the detail levels where it is needed, without forcing too much detail everywhere.

Another issue with maps with too much details is that they tend to look cluttered and unreadable when zoomed out, because of all the tiny details.

If you want to make them tiled, make sure that they are rotated with the steps. If a spiral staircase is , it would be because it is made from large stones dropped into a stair formation, and those would follow the rotation of the steps, not the alignment of the floor above or below. Same goes if you make them from planks.

Not playing either of them, but in my view, being a DM is all about taking the pieces you want for your own campaign and mashing them together, no need to feel locked into one particular world or setting.

Symbols keep their layer when defined, and the entities should normally be put on the SYMBOL DEFINITION layer before you define them, this won't happen automatically. As for the Common sheet, that doesn't really matter that much, since sheet information is lost when defining the symbols, but having all entities on the same sheet is a good way of seeing that things look right, otherwise the actual entity order in the finished symbol may not be as you percive it since sheet ordering trumps entity ordering. Using Common is a good practice, because Common isn't a sheet at all, it is just all the entities that doesn't actually have a sheet, which is exactly the situation for a symbol definition.

For the catalog, yes. A good way to build a custom catalog wich includes existing symbols is to first make a catalog with your symbols, using save as catalog from the symbol manager. Then, open this new catalog up as a map (as opposed to in the symbol catalog window), and then open up the catalog with the symbols you want to insert in the symbol catalog window, clicking on each symbol once to import it into your own catalog. Then simply save it (using the normal save button this time, no need to to the symbol manager route every time).

If they are pixelated, it is usually because the scale for the fill in the fill style dialog is way to large. Try reducing it for the fill styles in question.

You can also force CC3+ to use a higher resolution version of the file through the Display Speed Settings.