Horizon of Darkness

Abilities are assumed to be only active, without passives (because how should they then interact with the scheme from the table)?

They can also be passive. But in that case, you will have to reserve a cell for the passive ability, and it will not be possible to replace it.
The scheme in the table serves as a kind of template so that you at least roughly understand what damage your abilities will inflict. But you can follow the scheme directly if you don't want to come up with something special.

What about restrictions on equipment? For example, does heavy armor reduce speed/dodge and everything else?

I will do it that way. The scheme is approximately the same for weapons.

Bad armor: -5% damage and speed, requirement: 2 units of strength
So-so armor: -10% damage and speed, requirement: 3 units of strength
Slightly better armor: -15% damage and speed, requirement: 4 units of strength
Not bad armor: -20% damage and speed, requirement: 6 units of strength
Good armor: -25% damage and speed, requirement: 8 units of strength
Very good armor: -30% damage and speed, requirement: 10 units of strength
Even better armor: -35% damage and speed, requirement: 13 units of strength
Excellent armor: -40% damage and speed, requirement: 16 units of strength
Super armor: -45% damage and speed, requirement: 20 units of strength
Mega super imba armor: -55% damage and speed, requirement: 30 units of strength

Speed - is it the number of steps in combat?

Yes.

Added 2 minutes ago

Can you play as a sentient sword?

In principle, yes

Also, instead of stones, they sometimes use weapons. Legend has it that when a soul of a relative enters it, it strengthens the item. Usually, warriors themselves bequeath to place their soul in a blade so that even after death, they can strike down enemies. And some enchant their weapons so that after death in battle, their soul immediately moves there.