I understand the lack of interesting in a Lego and Templar. When your job is to get beat up that isn't all that fun for example. I take a certain sick please in standing amidst my foes and laughing as they can't kill me but it is a passive role. I don't think that is about L2P. That is just that being a tank is often boring.
It is L2P for you, because neither Lego nor Templar is about absolute tanking on Perilous. Legos and Templars who try to shoe-horn into the 'tank' role are noobs, because Perilous difficulty DAMP isn't about tanking even with the invincibility skills around.
How to play those classes effectively is the complete opposite of your expectation here. I posted in 'good' advice I expect players to scream Leeroy Jenkins, tongue-in-cheek... but in actuality the skilled players are performing Leeroys successfully jumping into the fray of battle. Legos should actively engage everything they can, jumping from target to target. Templars combo off of wrath of heaven / spell purge to destroy everything not physical immune in 2 seconds, and then shield-wall attack animation cancel everything else in short time.
Playing these classes is not boring, your expectation of how these classes are played is boring, and you need to change your attitude if you are going to have any fun at all.
there should at least be a reasonably obvious path to victory with a class.
There is, hinted within the character tree itself. I've personally played L1 Katari in Perilous in a team environment, and sometimes even top the scoreboard with non-burst dps classes around.
If you're on this board, you have no excuses though since there's multiple Katari & Reaver guide
threads, with my own included. Outside this board, it is more intuitive looking at the easy to grab skills: Charge, Mighty Blow, Pommel Strike, Combat Roll. SP is also available to test skills, though not absolutely necessary. Looking at the bottom skills of Reaver is also pretty intuitive, though Warhorn definitely is not.
The game is balanced around routine difficulty and charge alone will bulldoze through routine AND threatening. It is really hard to have a completely borked build on Katari when charge is so good, just shades of min-max.