Some thoughts regarding the combat...
Let's say you have a fighter X.
In the first scenario he is facing a single enemy Y.
....
.XY.
....They are facing each other, have their weapons in one and shields in the other hand.
X has luck or is more agile or whatever and can lead the first turn.
First turn planning:X decides to do a low low horizontal swing from left to right with his sword, followed by a diagonal swing upwards from right to left.
Y has now a chance, depending on his experience and knowledge about fighting with swords, to see from X's movements what he plans to do.
That can be more or less successful, and Y can, to a degree, consider X's actions when planning his own actions this turn.
Let's say, Y knows that X is gonna attack low, not that it's going to be swing, just that it's low.
He now could decide to lower his shield, preparing to defend a low attack, and let follow a high slash towards X.
First turn action:X makes a low swing with his sword, aiming for Y's legs, but the attack is blocked by Y's shield.
Y reacts with a high slash, but X manages to parry the attack with a vertical swing upwards.
There would be calculations about the power of the attacks and who leads the next turn.
That could also include the mentioned advantage.
X advantage would slightly increase if he can make Y stumble from the power of his second attack.
Y advantage would slightly sink, thus he has a lower chance to successfully foresee and counter X's actions,
while X would have a slight bonus on foreseeing Y's actions and countering them because of a slightly increased advantage.
Falling to the ground would be a much higher change in the advantages and probably mean ones definite dead in the next turn.
--- ---A fight with more participants would be no problem, either.
.A..
.XB.
.C..X is surrounded by A, B and C.
This time, X isn't so lucky and A leads the first round.
First turn planning:A decides to make a high horizontal swing and nothing else, which could increase the power of this single action.
Since X is better than B and C, it's now his turn to plan.
He foresees that A is doing "something", but has no idea what exactly.
He decides to lower the shield in his left hand, to duck and to make a low swing from the left to the right with his sword.
Since he chooes 3 actions, each action is slightly less effective.
Now B plans. He doesn't manage foresee what X is gonna do and decides to try a high slash towards X's head.
C can't foresee what X is going to do, either, and plans to make a diagonal downward swing from right to left.
First turn action:A attacks with a high horizontal swing, but it is blocked by X's raised shield.
X cringes and makes a low swing, which hits B's legs,
making him stumble and almost fall (medium advantage loss for B and he is too distracted to do his action).
C attacks, hitting X's shoulder, but the hit is weakened by X's iron shoulder plate.
--- ---I'm sure there are flaws and gaps in this, but I think it could work and make some interesting fight scenes possible.
A fighter has the chance to foresee all actions planned before it's his turn to plan,
So if A, B and C would all have their turns before X, he could plan even better, like raising his shield, then taking an escape jump backwards,
avoiding the attacks he didn't block with a bit of luck.
He would have a disadvantage though, because he has to share his focus to the three of them, making it possible for him to not foresee anything or maybe even make wrong assumptions.
I think fights with more than 2 factions would work with this, too.
I hope I could inspire you a bit.
It looks really great what you have done so far, keep up the good work!