Stunning strike is ridiculously powerful. At 5th level a monk can force an enemy to make 4 saves to avoid being stunned. That is like triple disadvantage - if you need all 4 - which you might not. You might get thst stun on the first attempt - but you can have up to three more chances if you need it.
Oh, but the DC will be lower than the wizard, right? For many monks, that DC is only going to be 1 less than a wizard of the same level. So that would be once in 20 times that DC difference makes a difference. But you have to hit and that is far from guaranteed.... except it isn't hard to make sure a monk hits. For example, my 5th level monk was charging into combats with a Bless and advantage from several sources on a regular basis at 5th level.
The utility of it will depend upon the DM to an extent, but a strategic player in a typical game that is not designed to negate the monk can consistently neutralize one (or more) of the most significant threats in a combat.
To that end - I consider the monk's ability to use Stunning Strike to be the equivalent of the benefit of a full spellcaster.
You’re gonna run out of ki fast if you’re using it on every attack and
using flurry regularly, but yeah it’s very good while you have ki to spend.
Full casting, though, I think is too much. You have to combine extra attack, martial arts, flurry of blows, and stunning strike, to reach that.
Perhaps at level 1 you choose between 3 types of “mystic” you are.
Swordmage - You get Int to AC, a mark ability that can be used offensively or protectively, and a small spell list you cast at 1/spell level, at a half caster rate, and ritual casting. Can deflect any spell attack and either boost their AC until start of next turn or add damage of the same type to their next attack.
Monk - as phb, except we replace stunning strike with a restrained effect, and a few other options like a “if the target moves more than 5 ft before the end of your next turn, they take XYZ damage”, and a force damage “melee weapon attack with a range of 30ft”, and an iron fist style “big single strike”, and we beef up deflect arrows to be deflect attacks. Also martial arts attack doesn’t have to be unarmed, but it’s the same damage regardless.
Martial Savant - Strength or Dex based, gets some maneuvers, no stunning strike or other really mystical stuff, but can performs feats of physical prowess. Also gets the same change to martial arts as the monk.