One of the best things about Mystics is their versatility through using different disciplines depending on what is needed for a given combat. As such, I recommend a different tactic depending on what discipline you intend to use.
Brute Force: Grappling
Order of the Immortal mystics can be incredibly strong grapplers (even Dexterity-based ones) because of the Brute Force Discipline which has the Psychic Focus:
While focused on this discipline, you have advantage on Strength (Athletics) checks.
You can then use psi for Feat of Strength to become an even stronger grappler:
As a bonus action, you gain a +5 bonus to Strength checks until the end of your next turn.
Grappling is quite strong for tank builds as you can isolate melee targets so they can only attack you.
Celerity: Rope
Celerity is one of the better options for a tank Mystic thanks to Agile Defense which says:
As a bonus action, you take the Dodge action.
There isn't anything particular useful for your off hand, but there could be situations where you could leverage rope. Because you have Surge of Speed:
As a bonus action, you gain two benefits until the end of the current
turn: you don’t provoke opportunity attacks, and you have a climbing speed equal to your walking speed.
...you are quite capable of striking an enemy before rapidly climbing to a more defensible area. You can then drop the rope that you are holding onto to allow your less swift allies to more readily climb as well.
Diminution: Net
The Diminution Discipline is only particularly useful for using stealth until level 9 when it becomes incredibly potent for tanks (assuming you have some way to protect your allies rather than just yourself).
Once you reach level 9, if you are leveraging Microscopic Form:
As a bonus action, you become smaller than Tiny
until your concentration ends. While this size,
you gain... a +5 bonus to AC... and you can’t make weapon attacks.
...you will likely be using a Psionic Talent to attack, so your hands don't really matter.
Until that point, however, something like a net could be incredibly potent. After using Toppling Shift to knock a target prone:
As a bonus action, you shift to an incredibly small size and then
suddenly return to normal, sending an opponent flying backward... It must succeed on a Strength saving throw or be knocked prone.
You can attack with the net to restrain the target, preventing them from standing up and allowing for future melee attacks to be made with advantage.
Iron Durability: Two Weapon Fighting
This is the "go to" Discipline for tank Mystics, although I suspect you will still get more mileage out of Celerity at middle levels, and Diminution at higher levels.
That being said, the flexibility of Iron Hide as a reaction option can be quite potent when you don't have as many psi to spend at early levels.
As a reaction when you are hit by an attack, you gain a +1 bonus to AC
for each psi point you spend on this ability...
However, I believe that Steel Hide is not particularly useful. By the time you have enough psi to be using this every turn, most enemies will be incorporating other damage types, and you'd be better off with Celerity for attackers, or Adaptive Body for elemental types.
As such, I'd just go with Two Weapon Fighting. If you absolutely need to, you can still use Steel Hide, but most of the time, your bonus action will be free to make that second attack for a bit more damage.
Psionic Weapon: Caltrops
This discipline carries the best damage output of the rest, especially starting at 6th level (until then Brute Force almost keeps up). With that in mind, I'm not sure just how often a tank-focused build will want to use it.
However, if the opportunity presents itself, it can be used to output quite a bit of burst damage via Lethal Strike:
As a bonus action, you imbue a weapon you’re holding or your unarmed
strike with psychic energy. The next time you hit with it before the end of your turn, it deals an extra 1d10 psychic damage per psi point spent.
One fun tactic is to use this during a surprise round to help take out a single enemy, before changing disciplines to something like Iron Durability or Adaptive Body (where you can still use your reaction) on the following turn.
For these situations, I recommend carrying caltrops. This way, if the party does get caught while sneaking, you can drop the caltrops on your first turn for when the group wants to retreat to a more advantageous position considering the original plan was thwarted.
The Rest: Two Weapon Fighting
The rest of the Disciplines either don't have bonus action options, or they come up very sparingly. You could always fall back on some of the other items I mentioned, like a rope to pair with the climbing speed from Bestial Form. However, without a regular bonus action, these niche cases are insufficient compared to just using Two Weapon Fighting.
Get that little bit of extra damage, and remember that you can always switch that off-hand weapon out for something more useful if the battle scenario lends itself to a different tactic.
