Playing around with the bitmap scroll demo, it didn't seem horribly slow & the color sacrifices in character mode were pretty bad.  It could have a 2x scrolling mode.  But the low fidelity doesn't justify more than character mode.  The Blue Max strategy of changing color palettes over water & land or based on current tile would eliminate the need for more colors.  A text file defining rectangles with the background colors might work.

The problem is the amount of time required to make the most minimal game artwork is not on a path to ever being finished.  Just a rough SDF-1 sketch with no elevation information, no greeblies, not drawn to scale took 2 hours.  Overlaying TV show art or freepawing before drawing character line segments seems to help. 

The map evolved to constrained line art on an 8x8 grid.  A character set with just line segments of different slopes & a drawing system based on these constraints might work. Line intersections are difficult.  

Facing the prospect of converting the complicated island outline to line segments, it became clear that a custom drawing program for drawing constrained lines on top of a photo is required or it's never going to be finished.  The constrained wireframe image can then be loaded in Gimp to fill colors.

There's no doubt if it paged instead of scrolled, RLE compressed bitmaps would be the way to go.  A modern game engine could just copy assets from the TV show.  Goog uses parallax distortion so copying their maps won't work.

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The ideal way to store a map would be raw sector access.  The commodore could access any sector on disk the same as a modern block device. The other formats either didn't have random access or reserved a lot of memory for an inode & directory, which isn't very efficient if you're only reading.

It seems concurrent disk I/O while running a program was achieved with a serial port interrupt handler.  It wasn't free.

The key need is an algorithm which compresses a PNG into an optimum character set & color map. 

Using character graphics with its limited colors simplifies the map compiler.  It needs to just match every 8x8 with a growing character set.  The 3 background colors are fixed.  It only needs to assign the 4th color to color memory.  Then it needs to write a double size proof showing the character codes in every 16x16 so the user can fix bugs.

If it used multicolor bitmap mode, the map compiler would have to search for an existing character which could match the current 8x8 by reassigning 3 colors.  It could save memory.

Because of the limitations on Y scrolling, smooth scrolling would be only in the X direction.  There are ways to use the raster interrupt to scroll both ways in Y, but they reduce vertical resolution to 23 rows.  It's doubtful if reducing X to 38 rows is worth it, since lower frame rates & choppy scrolling have gotten normal in the last 40 years.

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They achieved vertical scrolling by showing a score panel on the blanking area.  That also had a large looping map.

This world map looked like 100% bitmap drawing with no page flipping.