This version of the Way of the Astral Self should bring it in-line with other martial classes without effortlessly surpassing them (and making them mad at you for bringing busted homebrew). To keep up with a group of min-maxers, you should ask your DM to also include a monk item that can give +2 or +3 to unarmed attacks at high levels. That will give this or any good monk build the kind of boosts that we lack from really popular feats. There are plenty of handwraps, bracers, or rings on the market that can do this. This subclass works best with a single base class change listed below, which should smooth out monk progression. Lastly, I made the AoE from summoning the astral arms scale with levels. Instead of summoning parts individually, you spend the amount of ki for your level, summon all available parts at once, and release an AoE that gets larger and more powerful as you level to stay relevant. It can also be recast while you have the form active if the opportunity arises. At level 17+, it should act similarly to a Paladin's destructive wave spell with a little less range and damage since it is a bonus action. I felt this was a golden opportunity to give another martial a modest amount of AoE damage.

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.

Arms of the Astral Self

3rd-level Way of the Astral Self feature

Your mastery of your ki allows you to summon a portion of your astral self. As a bonus action, you can spend 2 ki points to summon the arms of your astral self. When you do so, each creature of your choice that you can see within 10 feet of you must succeed on a Dexterity saving throw or take force damage equal to 2 rolls of your Martial Arts die. You can re-summon the arms even if you already have them to repeat this effect. As your Martial Arts die evolves and your astral form becomes more complete, the summoning will cost more ki, and the effect will have a greater range and larger number of Martial Arts dice:

• Level 3: Astral Arms, 2 ki, 2 Martial Arts dice in a 10 ft. radius.
• Level 6: Astral Arms and Visage, 3 ki, 3 Martial Arts dice in a 15 ft. radius.
• Level 11: Astral Body, 4 ki, 4 Martial Arts dice in a 20 ft. radius.
• Level 17: Awakened Astral Body, 5 ki, 5 Martial Arts dice in a 25 ft. radius.

 

For 10 minutes, these spectral arms hover near your shoulders or surround your arms (your choice). You determine the arms’ appearance, and they vanish early if you are incapacitated or die. While the spectral arms are present, you gain the following benefits:

• You can use your Wisdom modifier in place of your Strength modifier when making Strength checks and Strength saving throws.
• You can use the spectral arms to make unarmed strikes.
• When you make an unarmed strike with the arms on your turn, your reach for it is 5 feet greater than normal.
• The unarmed strikes you make with the arms can use your Wisdom modifier in place of your Strength or Dexterity modifier for the attack and damage rolls, and their damage type is force.

Visage of the Astral Self

6th-level Way of the Astral Self feature

You can summon the visage of your astral self. As a bonus action, you now spend 3 ki points to summon your astral form, including the arms and the visage of your astral self for 10 minutes. It vanishes early if you are incapacitated or die.

The spectral visage covers your face like a helmet or mask. You determine its appearance.

While the spectral visage is present, you gain the following benefits.

Astral Sight. You can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.

Wisdom of the Spirit. You have advantage on Wisdom (Insight) and Charisma (Intimidation) checks.

Word of the Spirit. When you speak, you can direct your words to a creature of your choice that you can see within 60 feet of you, making it so only that creature can hear you. Alternatively, you can amplify your voice so that all creatures within 600 feet can hear you.

Ki Visualization. You can see Ki leaving the bodies of fallen foes and attempt to grasp it. When a creature within 10 feet of you is reduced to 0 hit points, you can use your reaction to regain 2 ki points. This increases to 3 ki points at level 11 and 4 ki points at level 17.

Body of the Astral Self

11th-level Way of the Astral Self feature

As a bonus action, you now spend 4 ki points to summon the entire Body of your Astral Self. This spectral body covers your physical form like a suit of armor, connecting with the arms and visage. You determine its appearance.

While the spectral body is present, you gain the following benefits.

Deflect Energy. When you take acid, cold, fire, force, lightning, or thunder damage, you can use your reaction to deflect it. When you do so, the damage you take is reduced by 1d10 + your Wisdom modifier (minimum reduction of 1).

Astral Barrage. Whenever you use the Extra Attack feature to attack twice, you can instead attack three times if all the attacks are made with your astral arms.

Awakened Astral Self

17th-level Way of the Astral Self feature

Your connection to your astral self is complete, allowing you to unleash its full potential. As a bonus action, you now spend 5 ki points to summon the arms, visage, and body of your astral self and awaken it for 10 minutes. This awakening ends early if you are incapacitated or die.

While your astral self is awakened, you gain the following benefits.

Armor of the Spirit. You gain a +2 bonus to Armor Class.

Empowered Arms. Once on each of your turns when you hit a target with the Arms of the Astral Self, you can deal extra damage to the target equal to 2 of your Martial Arts dice. In addition, whenever you use a bonus action for Flurry of Blows, Step of the Wind, Patient Defense, or Ki-Fueled Attack, you may make 1 extra attack with your astral arms.