A tip for mining Copper that I use and have had almost no problems with mobs. At least it did as of Nov 2021 when I last played the game. The devs might have just set the spwn rate insanly high.

For the Copper the first thing you do is dig down as far as you can just next to the deposit and then work your way all the way around the deposit. As long as the walls are sheer the mobs won't just drop into the trench and attack you. You can also put gates to secure the area your working in and when you start diggin into the copper you can drop a camp fire and keep your rested buff thanks to the Copper deposit above you counting as shelter.

Unless it was changed recently just digging in dirt doesn't attract mobs, its only when you start hitting rocks or trees that things come running. If you have a good trench the mobs can't get at you or you at least have them coming at you from one direction.

Also works for Silver.

Hope this is useful. Good luck.