No, you can't just learn all spells once you're a certain level.
Some Master skills require level 12, some level 15.
You can increase your Int by making staves using Crafting 5 that have +2 Int. I don't know about wands, I don't use them.
I have heard that you will really need Willpower and Bodybuilding mid to end game. I haven't had much need for them so far (level 13), but I've made sure to put some points into them on all 4 characters.
Another way to boost damage is by increasing Leadership on a character. One of my mains has Leadership 5 with +10% damage boost for the other 3 characters. The other main has Leadership 4 which gives the other main +10% damage.
