I agree with FrankW's & genericname's assessment above.
One other idea to keep in mind is that most large format printers were engineered to output signage that will ultimately be viewed from at least 5ft away.
The graininess tends to be more evident in lighter colors, so if you have creative freedom for those files you may want to change the colors.
Printing in CMYLcLmK may improve it as well.
When making perforated vinyl window decals, our printers almost run in draft mode, and the output is good- when making small, detailed prints like yours, the pass count is much higher, the resolution is higher, the ink levels are slightly higher and the print mode is unidirectional.
Good Luck!