It seems the most logical conclusion at this current time that a pure heavy attack damage setup for pretty much all classes ends up in a medium armor setup with Storm Master and the heavy armor set Sergeant's Mail in the form of jewelry and weapon.
It also helps that the main item sets are very easily farmable by players of any level!
With Oakensoul as a Mythic item that restricts other Mythics from being used and also restricting the usage of a second skill bar, we don't have too many options in terms of set availability and also makes us resort to a 1x bonus set such as Slimecraw's 1x Critical Chance bonus.
After some testing on both the raid dummies and in content (also based on previous testing with a different iteration of Oakensoul), it doesn't seem the Berserker enchant (Weapon/Spell Damage increase) will outperform either Poison or Flame Damage enchantments on the Lightning Staff.
Jewelry enchantments can be Weapon or Spell Damage glyphs because even if you have a Dragonknight or not in your group that provides Minor Brutality, you as a Templar provide yourself Minor Sorcery from casting any Dawn's Wrath ability.
An alternative body setup can be 5x Noble Duelist's Silk or 5x Order's Wrath instead of Storm Master for solo instances/arenas or 4 man content!
The Templar introduces some risk adding another channeled attack on top of a couple channeled abilities already. However the payoff in terms of damage during execute is nice and the inbuilt healing is quite nice to help the class out.
For your opening rotation, cast Shooting Star ultimate on target then immediately cast Barbed Trap without pause on target.
Cast Power of the Light as Shooting Star lands then start Heavy Attacking into a Ritual of Retribution cast.
After, heavy attack again into a Power of the Light cast then heavy attack weave Puncturing Sweeps and simply keep up all your damage over time effects like Barbed Trap, Power of the Light and Ritual of Retribution.
Once you reach 27% of the target's health - unless you get a good window of opportunity at 30% health if you get Off Balance - start heavy attack weaving Radiant Oppression while keeping up only Ritual of Retribution and Barbed Trap. Stop casting Power of the Light at this point as well.
Once you reach 15% of the target's health, stop recasting any other ability including Barbed Trap and Ritual of Retribution. Instead of heavy attacking Radiant Oppression, you must Light Attack weave Radiant Oppression until the target is dead.
Radiant Oppression can be replaced by Radiant Glory which does additional healing for the damage caused when you start beaming targets.
Crescent Sweep was tested instead of Shooting Star but it's significantly less burst damage.
You can use Barbed Trap as a flex skill slot as well to substitute it for something else you need depending on content.
Champion Points Review
Celerity is important to help you move out of danger as you channel heavy attacks. Bastion is a flex slottable, meaning you can slot something else you need if not casting a shield, such as Expert Evasion for a free Roll Dodge.
This build can be used for a variety of content including overland, solo Arenas such as Veteran Maelstrom Arena & Vateshran Hollow, veteran dungeons and Veteran Trials.
Recommended race: Altmer (High Elf) or Dunmer (Dark Elf); for solo or 4-man content you can go Khajiit.
The Altmer is recommended due to its passive for reduced incoming damage while channeling abilities as a Templar.
If you want to go with 64 points into Stamina instead with a different race like Orc or still even with Dunmer you can, at that point it just depends on the usage of your resource more so than the damage of your resource such as casting utility skills based on Stamina, casting Stamina based healing like Resolving Vigor or performing Roll Dodges and more Blocks.
Recommended food will be a bi-stat Health and Magicka food. For parsing or for fights where you don't need that extra health, you can use purely single-stat Max Magicka food! For soloing veteran dungeons, you may need to invest into a tri-stat food (Max Health, Max Stamina, Magicka)!
Recommended Mundus will be The Thief (for group content like Trials) or The Lover (for solo content and overland).
Potions can be your basic tri-stat restoration potions, however if you'd like to invest further into the build, you could go with Resistance Potions that restore 9206 Health immediately & gives Major Fortitude (30% Health Recovery), grants a unique 5280 Spell and Physical Resistances!
You should have about 25,000 resistances, making you basically a tank with around 30,000 Health (includes Minor Toughness if in group with a Warden).
The potion can be made using the following reagents: Mudcrab Chitin, Mountain Flower, Bugloss