I'm taking gears into consideration since +attribute gears appears to be common. It surely need more balance and testing with gears taken into consideration. Too less and it kills the system, too much and everyone has 6*15 is not good, either.
You're not even providing 1 * 15 base attributes, so I think there's some wiggle room to boost the amount of points you give. Even at 5 attribute points at level 1, and 1 per level for the next 19 levels, that's 24 points, or enough to max out two of the 6 with a little wiggle room.
I also dislike your point distribution because it leads to empty levels. There is literally nothing to do at level 8, 12, and 16, and only one thing to allocate at levels 2, 4, 6, 10, 18 and 20. Other levels are feasts: 5, 7, 9, 11. and 13 have three things to allocate, and 17 has four.
Steady progression is better than a feast-and-famine approach and allows one to smooth over trouble spots or change things more easily.