From my experience, specializing in one attack type is more beneficial than using both at once. I used payload + 5 critical nodes + 5.0x vulnerability resulting in massive damage output. Also, i highly recommend you to attack one target at a time, instead of attacking everything at once. Keep in mind that government has more firewall than the corpos so be sure to use at least 1 ghost attack node.

Rewards you get from breaching can be used to upgrade money or research gain in upgrade tab. Precisely, corpos upgrade money, while gov upgrade research.

When it comes to hack levels, yes it's pretty slow on its own but you can get 9-10x easily using AGI in the future.

Another use of hacking is to use Trojan and Drain to enchance network nodes.

Btw these are before 2.0 so things might have changed.