This monk subclass is not very original according to any search of the homebrew listings which contains the word "iron". It is meant to be a heavy unarmed hitter that may feel similar to a well-armed fighter or barbarian with a predictable melee attack rotation that uses situational and strategic resource expense. However, I hope you will find the combination of mechanics to be interesting without being over-tuned and the flavor to be very enjoyable. While it should be fun for any type of character, it adds a good flavor niche for especially large characters like goliath, loxodon, and firbolg monks.

General Notes: I would suggest only a single change to the base monk class, which is that (ki = proficiency bonus * 3) until level 20, when you just raise it from 18 to 20 ki. That should smooth out the progression somewhat, allowing monks to spend more time in a sweet spot for endurance but not over buffing what they can do with the ki.

Body of Iron

Starting when you choose this tradition at 3rd level, you gain the following benefits:

Lead Feet. You plant yourself in a very firm stance to increase the power of your Unarmed Strikes. You may sacrifice 15 ft of movement to attack with Unarmed as an Iron Strike, adding 5 damage. You may make multiple Iron Strikes in a single turn with Extra Attack or Flurry of Blows if you have enough movement to spend on each one. The stance must be taken and the movement is declared spent before rolling a d20 to hit. You can't make this type of attack when you have no movement to spend such as an opportunity attack.

Iron Endurance. You gain resistances based on the harsh conditions under which you trained.

Catching the Tempest at Vorde

You have gained resistance to Thunder and Lightning damage.

Following the Voice in the Al'Ishtal Desert

You have gained resistance to Radiant and Psychic damage.

Hunting in the Deadmire

You have gained resistance to Necrotic and Acid damage.

Scaling the Mountains of Shan'La

You have gained resistance to Cold and Force damage.

Unbreakable Fists

At 6th level, your fists become like iron, granting the following benefits:

Problem Solving Fists. You have proficiency in intimidation by cracking your knuckles. For the cost of 1 ki, you can use your hardened fists for any task requiring thieve's tools with expertise, such as lock picking or disarming traps by punching them. Make a dexterity or tool check with expertise. You utterly destroy any mechanical traps or triggers on a success. If you use this to unlock a chest that turns out to be a mimic or any object that was animated and waiting to attack, then it is surprised for the first round of combat.

Deflect Attack. Your Deflect Missiles ability now works against melee attacks. You can use your reaction to deflect or catch a weapon, fist, claw, or fang when you are hit by a melee weapon attack. You reduce the damage by 1d10 + your Dexterity modifier + your monk level. If you reduce the damage to 0, you can spend a ki to impose disadvantage on attack rolls until the end of the enemy’s next turn or spend 3 ki points to disarm the enemy if it is using a melee weapon (not unarmed or natural weapons). You may only deflect a number of attacks equal to your proficiency bonus per long rest. You cannot deflect attacks from creatures which are 2 sizes larger than you.

Iron Devastation

Beginning at 11th level, your unarmed attacks score a critical hit on a roll of 19-20 and on a roll of 18-20 against enemies which are stunned, restrained, or in any way incapacitated. You additionally regain 1 ki when you score a critical hit.

Unstoppable Force

At 17th level, when you score your first critical hit of the adventuring day with an Unarmed Strike, you roll an additional martial arts die for a total of 3. Each successive critical hit adds an additional die. This effect resets when your party takes a long or short rest.