Did you read the thread I linked? For example this post: Beginner's questions before sending for print - #13 by eelik ?
Or much more. You could have vias for example in a rough 2 cm or 1 cm grid everywhere where they don’t touch other nets, and then vias in all sharpish corners of filled GND areas, and then along all edges of the filled areas. This may be exaggeration but probably there still wouldn’t bee too many vias. The point is that copper isn’t really 0 ohm resistor, while a good GND has as close to 0 ohm as possible between any two locations. (As a rule of thumb; I’m not a specialist, so take this with a grain of salt.)