5e knight: Fighter subclass

[Blackflight]

I like the ideas of these, though I'd be inclined to tweak them slightly:

Shield bash: While wielding a shield, whenever you take the attack action you can make a single attack using your shield as a bonus action. You are proficient with this attack and it deals d4 + your strength modifier as bludgeoning damage.
^ line it up with the same form of action economy as TWF or PAM

Parry: While wielding a two-handed melee weapon you gain a +2 AC bonus against melee weapon attacks made by creatures within your reach.
^ less tracking involved.

Now questions about shield bashing:

• Would shield bash's to hit and damage rolls be modified by a +1/+2/+3 shield?
• Would a modified shield (MM p204: Lizardfolk's spiked shield) override the 1d4 bludgeoning damage?
• Does treating a shield as a weapon negate the Dueling fighting style?
• Is treating a shield as a weapon compatible with the Dual Wielder feat?

I like these suggestions. I initially had shield bash as a bonus action but thought it would ruin the action economy of knight´s challenge and the Maneuvering command, but on second thought, having it as a straight up extra attack is perhaps a bit overpowered.
And about your four questions, I´d have to think more on these. I don´t want to cripple the duelist choice but then again, it is effectively a weapon with this feature. Maybe it should be optional to use it as a weapon or not.

I agree that there is too much tracking involved in the way I wrote parry. But if it applies to all melee attacks would it be better to have parry as a reaction to also tax its action economy or is it fine as is?

And speaking of tracking, do you think its too much work to keep track on what creatures are immune to knights challenge (for being previously challenged)? I could make it so that you can use it multiple times at the same creature but with a limited use per day (similar to Commanding presence) -> A variant would be to have knight´s challenge take up a use of your commanding presence and instead make commanding presence recharge on each short rest.