I got it! The trick is to use success counting rather than summing. That allows for fewer dice, too: start with just one point in each attribute, and distribute three more at will. Roll as many d6 as you have points in each attribute. Each 5 or 6 on a die is a success. In opposed rolls, you compare the number of successes. This should work!
Now for combat. I want the point balance to matter, so in a straight fight both opponents roll Grace. Whoever comes on top now rolls Power, and deal as much damage as they get successes. Plenty of room for variation here, like the initial margin of success (from Grace) being used as an extra damage die, or even automatic damage. Anyway.
(Your Power score would also indicate how much damage you can take without collapsing. So a wimpy wizard with just one die in Power would have 2 HP basically.)
The best part? From here on a lot of variation is possible. Like a "charge" action: roll Power directly, and deal damage accordingly. But! The opponent gets to roll Grace unopposed and otherwise handle it like a normal attack. That makes an extreme build with all the points in Power viable, but not overpowered.
As for casting? Roll Focus against the opponent's Grace. If they win, they roll for damage as above. If you win, the margin of success can be used in various ways:
- deal automatic damage, rationalized as magic missiles, throwing stars or whatever fits;
- gain a magic shield that absorbs some damage before fizzling out;
- gain energy, that can be used on subsequent combat rounds to pay for rerolls, extra dice to roll or the like.
To keep spellcasters from just spamming their special attacks, make each success on a Focus roll cost a magic point from a pool; once this pool is depleted, the caster takes damage instead.
As for choosing the strategy of enemy NPCs? Remember that my original goal was to do without a GM or AI when it's not practical to have one, such as in a gamebook. Hence why the default is to roll for attack (with Grace) in response to pretty much any action. But you could also have special enemies, like a charging bull that always, well, charges. Or an evil wizard with a big pool of magic points who keeps shooting magic missiles.
(Of course, magic points could slowly replenish during combat, one per turn or by other means.)
That's all I have for now. None of this has been tested yet, but seems sound and should work. Feedback would be most welcome.