no really good places because people pk or steal your shells so i agree that car missions are good. If you do want to dig maybe below that shop that sells fences and trampoline and stuff
My favourite digging spot is in the park near Tachi. If you have the required amount of hours, that's great. It filters out players with less hours plus it's pk protected. But it's a tight spot though.
Do car missions when there is less people. The more people that are doing car missions, the likelier chance a pker will come. If that happens, switch to the other car mission NPC. If that area has pkers then wait for a bit and trash/mushroom pick.
Digging at the end of the world is nice, if someone starts shooting you can grab your shells and run into the water or another good place is the holiday island in the back area.