Its still OP though, compare it to a clerics semi anemic efforts at making an attack which gets a whole 1d8 extra damage at level 8. A warlord granting at will attacks to a Rogue is bad enough but even Barbarians and Fighters let it deal a lot of damage (at will attacks as sharshooter, yes please) even using the bonus action/reaction.
A single level 8 barbarian attack does 2d6+7 = 14
A single level 8 cleric attack does 2d8+5 = 14
A single level 8 dragon sorcerer firebolt does 2d10+5 = 16
Granting a single attack is very much on par with using a cantrip.
You will, at most, deal 50% of whatever damage your allies deal. Usually less since you don't get to benefit from things like action surge, flurry of blows.
Really, your much better off giving a fighter advantage for 2 attacks (greater invisibility), then 1 additional attack (haste). Even more so if they have sharpshooter.
but where do you draw the line to make up for its better attacks via a better class?
Compaire it to what can already be done.
For instance...
Level 8 divine soul sorcerer can cast hastes for each battle a day (using 5 SP to create a slot), has 7 casts of healing word, and firebolt. Plus option for other spells, like fireball, raise dead, and such.
Level 11 divine soul can twin-haste all day. And level 17 can twin-greater invisibility all day.
I would remove the +2 AC and force them to use up all their actions each turn (so no firebolts) to compensate for no risk of exhaustion and a d8 hit die.
Also, rogues can use haste to attack and ready an attack, getting double sneak attack. Though that's a bit of a loophole, so i wouldn't balance around it.
So, a balanced level 8 Tactician might...
*grant an attack as an action (on their own turn like haste)
*grant movement as a bonus action (half-speed, but no OA).
*grant advantage on dex saves as a reaction
*Heal for 60 HP. Or 80 HP out of combat.
*Attack for 11 damage on his own (only if it doesn't grants an attack).
But still, a martial support class has plenty of room to explore, beyond simply 4e attack granting. As you say, this is a new edition.
So just use the warlord as a starting place, and expand to other ideas. Add in all the other non-magic, non-damage sub-classes you can think of.
Like maybe a smith. Who repairs everone's weapon armor between battles.
Or someone who can use intimidate to cause fear.
Or a non-magical bard.
etc...