Not that you will read this because it's long but here goes. He's weaker at the start of 2018 than the end of 3. Kratos is constantly suppressing himself out of fear of himself. Thor calls him out on it multiple times and Baldur was unimpressed with him to the point he couldn't believe it. Tyr himself helps Kratos bring out the God of War hidden in him. Hraezlyr and many opponents would be no match for young Kratos. Kratos himself references his greater power in 2018 and Ragnarok multiple times, particularly when he holds a conversation with Odin about Atreus, warning Odin of "meeting the god I once was." This could easily be explained away with Kratos referring to his past temperament but I use Thor and Tyr as gods that see Kratos is holding back. We don't see his full power (mentally and physically) until Valhalla. He had to grow back into himself, accept all his past actions, focus on the positive he always tried to bring despite what he thinks of himself, and surpass the traumas of his past to regain that strength and use it for "good." Most of 2018 is about Kratos' relationship with Atreus rather than it being about Kratos himself or his character change since the end of 3. He really only goes through any change in Ragnarok and it's mostly into a flat out hero instead of a flawed anti-hero. Valhalla actually tackles and finally answers the topic of his power with his old throne representing that power and him claiming it once again.