This fighter might be a former duelist, trained in the art of reading their opponent's intentions and capitalizing on even the smallest opening. Perhaps they belong to an order of martial artists who emphasize anticipation and redirection over raw power. Their fighting style is characterized by fluid movements and an almost preternatural ability to react to sudden changes in combat. Their opportunity attacks are swift and unexpected, turning an opponent's aggression against them.

Incoming weakness

You can now do a opportunity attack on a enemy that gets in your range 

Multiple weaknesses

You can now do as many opportunity attacks as you want without using your reaction 

Stronger opportunity

Your opportunity attacks now do 3d4 extra damage and the target goes to half speed for 3 turns 

Further opportunitys

You can now do opportunity attacks from 10ft away 

Tiredness opportunity

You can forgo all attacks next turn when doing a opportunity attack if you do that attack does 3d10 more damage and the target can’t move this round and has a minus 1 to every roll 

Messing with the action economy is usually a bad idea, as is trying everything into one mechanic.

Unlimited OAs with a 10' reach that also slows/stops your opponents as it gives considerable bonus damage seems a bit much.  On the flip side, if your fighter approaches the enemy and the enemy escapes by Fey Step or something all those features go for naught.

I have tried several times to recreate the Duelist NPC class from the old Dragon Magazine but it's pretty OP (which is why it was never a PC class).

Good luck with it!