Yeah, Te/Bo is no longer a thing now that the scion classes are out.
On the Japanese side, Phantom did a good job on making the Force and Techter obsolete. So much so that the developers had to create things like compound techniques in order to lure players back from Phantom. You'll be going on a treasure hunt if you're serious about being a Techter.
For sub class, since you used the Phantom class before, I would recommend Phantom as your Subclass, although I personally decided to go with the Etoile subclass after using both for a bit.
The first thing on the treasure hunt list is Nabarta Type-0. This is your block button, and you can perform a strong counter with a high chance of freezing the target (when possible) on a perfect guard, much like your Braver's counter move.
Next on the list is Marlu's client order. It requires level 50, but when you complete it you get Heavy Hammer and the three simple compound techniques. Heavy Hammer is your one-and-only all-purpose Techter Photon art allowing you to close gaps quickly and deal big hits to the target. I'm personally not all that impressed with Re Zandia or Re Formelgia, but I do use Re Barantsia regularly. This technique allows one to move around the battlefield quickly while constantly attacking whatever target aimed at with ice and light. It may not generate "big" damage numbers, but you do get a lot of them over time, so it's well worth it in any situation where being close to the target is a bad idea. Especially fun with a Kwadarka Orb and the S1: Photon Descent 2 enhancement. 10% of PP costs and a constant 5 PP per second even while the technique is active can really extend the firing time.
Then there is Tech Charge Parry (L) ring. At a full +20, it gives 0.6 seconds of guard frames whenever you charge a technique. With Phantom Tech Short Charge from the sub, this gives very similar results to Phantom's class-only stealth charge. Except it's guard frames instead of i-frames, so you're still vulnerable from the back. Knowing what I know now, I wouldn't recommend other rings until you can bind this to a unit. Frankly I have the combined version Defense Techniques bound to one of my units right now. Alternative Wand E Charge may be really tempting, but stick it out.
The big compound techniques give you occasional big burst damage almost for free. Either complete Klariskrays's orders or buy them from the photon drop exchange to get them. I bought mine early for photon spheres and didn't regret it. But if you do have the levels in any class, Klariskrays is likely the better option.
In terms of actual combat, basic attacks backed by Photonic Fury and Wand Lovers can actually out-DPS most of your techniques and possibly many other classes' PAs, while earning you PP instead of costing it. It is definitely viable to use Shifta, Deband, Zanverse, Megiverse, Zondeel, and Type Zero Nabarta as needed while relying on just basic attacks to kill opponents. The only trouble with this is it's also dangerous. You don't have a guard cancel so just getting into an opponent's face and swinging blindly is a very good way to get you smacked back in return. I find myself using Techniques for damage at range very often. It gets the job done safely most of the time.