I main and destroy people using Aganos on a regular basis. Many people underestimate him, or even better, fear him. Just the other day, I had 5 opponents all quit after the 1st match, all in a row. It gave me a good laugh.
It’s the fundamentals that you mentioned that are important. Knowing how to use his normals to great effect to keep your opponents away and your chunk armor to punish them if they get in to close is key. I find often being patient and waiting for the opponent to do things 1st is a great strategy. Having armor, Aganos can afford to wait longer than most and then counter effectively, because he can absorb the hit.
They shoot a projectile? Flick it, block for meter, or perform a shadow ruin (careful with the timing though!
They jump from far away? Wait and counter with a (shadow) ruin, a MP payload assault, sHP, sHK, sHP peacekeeper (if you have it), throw the peacekeeper to push them back, etc.!
They jump at you from medium range? Upper finger-flick!
They jump at you up close or try to cross-over? Any pulverize!
They jump at you from ANY range? Natural disaster!
They walk or dash towards you - poke’m (d+HK works great too for chunking up, if you need it)!
If they run away? Put a wall behind them!
They stand still? Chunk up and move in for the kill!
There are so many options with Aganos, it’s ridiculous! People really do not understand just how good he is - simply having his peacekeeper during instinct, which many people think is terrible, gives you numerous great options. It immediately gives him a near full-screen 50-50 low-overhead mix-up (simply switch between d+HP and HP) that can also be used to recapture or destory projectiles, get hard knockdowns, causes knockback, and gives him time to chunk up if needed. Sure, it’s not as great up-close, but his K normals are so good overall, it doesn’t matter!
Sure his ADs are easy to read and combo-break, but if they start breaking you, then you can either lock them out with a counter-breaker, or change it up after doing the same ADs for a short time to fake them into a lockout, or perform an additional special move after one (for example, natural disaster followed by another natural disaster) to mix things up. Either way, the ball’s in your court as Aganos.
One of my favorite things to do is to start a combo and end it with shadow payload assault - most of the time, the enemy doesn’t recover from the combo before the assault hits them. Once it does, you have enough time to chunk up, place a wall behind them, and then to smash them through it with ruin. This setup, sans combo, is worth a great deal of damage on its own (and then you add the combo or any additional walls on top of it)!
I don’t know if he’s top 5, but he’s definitely better than most people think he is in the right hands.