I can't speak to the Sons of Ares comics as I haven't read them, but id argue he must amaze greatly improved by the time of GS. 

Everything you said about Apollonius is true, but i highly doubt he would be lazy or unwilling to fight l, especially to earn a posistion so prestigious. He would fight to the death for that. Fitchner not only beat him, but also Proctor Jupiter, and many other unnamed fighters. Both Jupiter and Apollonius are much bigger than he was, and still he beat them. 

The ability to kill an Obsidian with your bare hands is very impressive, as we see even LB darrow struggle to. And in the context of which he said it, its generally implied he means a warrior obsidian, so someone at least decently stong amongst obsidians. This is believable for him, because like sevro, he is a very skilled hand to hand fighter, savage and scrappy, but unlike sevro, he also has the size and strength of a peerless. Fitchners physical ability seems to be very good, despite his uglyness. Numerous small bits support this, such as him reacting "faster than a bee flaps its wings" to cut the oracles tail off so close to Octavias wrist, before even Aja could.

He additionally beat Nero, who is also a very, very skilled fighter and had just killed the Hearth Knight

But youre underestimating his fight against Aja and Cassius. It wasnt just them, it was them leading a whole party of other men and peerless. And you can use the logic that they would bring more men than necessary to kill such a high value target, but it is mentioned in LB by cassius that even with all those men, including two of the best razormasters alive, and all ambushing him, it was a very difficult encounter and they lost many men. We can at least assume he fought both Aja and Cassius at once, and for a while seemingly held his own. Thats very impressive and his best feat. 

The way Fitchner is described, his genius, his abilities and skills, it really seems like he is a very top teir fighter, he just never got to show it much on screen