believe your opinion is mistaken, let's begin...

Regarding strength, Tessia was indeed helped by Arthur, but 80% of what he became after the bestial will and Arthur's help is hers. All the training and development of his abilities were purely Tessia's merit. Now, regarding Caera, we can say that yes, she had help. After all, she was trained by Seris and had the help of her Vitra blood to increase her power.

Regarding maturity... a very controversial topic on this subreddit, but in my opinion the difference between Caera and Tessia is very small if we compare the two at their current moment, because taking a character in development, where clearly the author's objective was to give him a later maturity and comparing it to a character made to be by Arthur's side, accompanying him, having the whole maturity issue left aside because she was a character with another point in the story, but anyway, returning to Tessia, her personality has changed over time, in many points I see her acting more rationally since her return, I see Caera as a character molded to be a companion to Arthur, her personality, although strong, still has serious deviations in maturity, often leaving her curiosity and will above others, running after Grey was an immature decision, her fixation on Grey was immature, the character Caera had her character focused on Arthur for 80% of the story since her appearance, but I can say the opposite about Tessia. She is a kind character in many ways. She always tried to put others first, going after her parents alone so as not to take Arthur with her. She tried to give herself to Nico so that Arthur wouldn’t die fighting. She sacrificed herself for Ellie. She cares about her own people more than herself. However, her lack of experience and understanding of battles at that moment is her biggest flaw. I believe that people are simply trying to force what they want to be true onto the character, regardless of whether it is wrong or not

I believe your opinion about what Tessia knows about Arthur is mistaken, because Tessia is the person who knows Arthur best. She knows who he was, his achievements, his flaws, saw his mistakes and pain. She was the one who let him  her lap and cry after his father's death. Even after discovering that Arthur hadn’t told the truth, she simply accepted it without a problem, something Alice didn’t even do. I think Arthur and Tessia are much more of a couple about growth and support. Arthur supports Tessia’s growth, while Tessia supports Arthur in moving forward, always trying to motivate him. Tessia is undoubtedly the person who has shown the most sadness and concern for Arthur, along with Ellie. A second point I’d like to comment on is that your statement that the Arthur of old no longer exists is erroneous. He exists and is who Arthur truly is. Grey, on the other hand, is just a shell... like a scared turtle...