I feel Phoenix Wright is among the characters who are like,
"If I didn't have constant anxiety, it'd be over for you."
He already seems impossible from the outside in canon when he's often a nervous wreck. Imagine if that anxiety wasn't there.
Take a look at these three sprites for a second. (It takes a little while, but I promise you'll see the relevance.) They're the confident/smiling forward facing for each person. And... there's some interesting similarities, yes? AJ-era Phoenix's sprite is pretty close to being a perfect mix of Godot and Mia's. His hands in his pockets, midway between Mia's crossed arms and Godot's hands on hips. They're all angling their bodies slightly to the side in the same direction. Mia's head is tilted the opposite way and forward a little; Godot's in line with his shoulders and back; Phoenix is in line with his shoulders but closer to Mia's angle. Heck, even color wise - it's definitely more of a stretch but Phoenix has on a dark jacket and necklace like Mia, but his hat is closer in color to Godot's shirt, and it is also noticeable/bright headwear like the mask.
The reason I bring up these sprites is because... well, in AJ we see Phoenix from the outside for the first time and he really does seem very inscrutable. Knowing him, he's definitely still feeling a lot of anxiety over things, but he doesn't show it and thus we the players don't see it, since he's no longer our POV. I think there are a couple factors at play there as well, because Phoenix certainly seems to have gotten better at hiding his flopsweat moments in that game. You could say there's a certain element of detachment, because despite all his machinations, in the end he isn't a lawyer for any part of that game and so ultimately, the responsibility is no longer his. Maybe that's a factor.
But AJ-era Phoenix (or 'Beanix') keeps coming to mind for me. I realized a few months ago that in that game, his relationship to Kristoph (and the plot itself as well as Apollo) strongly mimics Mia's with Dahlia. However, looking at these sprites got me thinking that he also mimics Godot.
AJ-era Phoenix also returns after a long absence from the legal world, which was forced upon him due to the actions of a poisoner who successfully tricked him (Godot actually got poisoned, Phoenix just disbarred). And when he next shows up in court, he is visibly very different (mask and hair vs. beanie/hoodie). He's also a lot more jaded, and takes an interest in a young new attorney closely linked to the poisoner responsible for his absence. He's secretive, often for what seems like no purpose at all, but has a plan that's only revealed in the final case - which itself was a combination of somewhat longterm planning/buildup, and adapting to unexpected situations right at the last minute. He takes risks and works outside the courtroom/sometimes the law to achieve his goal (the bloody ace isn't exactly the same as stabbing Misty Fey, but, y'know), and in general appears quite different from his past self. However, in the end his goals are very sympathetic, and much more than just simple 'revenge', however morally dubious some of his choices have been.
Obviously, Beanix is not one-to-one with Godot. But I can see a rough sketch of similarities there, and certainly in general attitude/vibe they're more similar than Phoenix and Mia seem to be at that point. Even some of the things Godot says seem like they could be a Beanix quote (this came to mind). I think there's at least enough to go on to say that AJ-era Phoenix is once again a mix of Mia and Godot.
I find this interesting because Phoenix looked up to Mia right from the beginning, and modeled so much of his behavior, strategies, and philosophies after her. And yet, she just as obviously learned a lot from Diego, back when she was the inexperienced new lawyer. The most notable being, of course, the quote passed down to Mia and then Phoenix: "A lawyer only cries when it's all over." (Another Godot quote that seems to apply to Beanix is this one.) And the thing is, Phoenix looks up to Mia so much, and she generally seems pretty knowledgeable and unflappable, but once we get inside her head/play a few cases as her we see she has a lot of nervousness hidden behind her facade, much like Phoenix. Of course, those cases were when she was a rookie attorney, but I think the point stands. Godot also reveals a heck of a lot of issues and self-recrimination later on, but for the most part he's really good at seeming very confident. It's another similarity between all of them.
And yet, Godot's problems are more personal. When it comes to the courtroom, he's actually pretty legitimately confident for the most part. At least, I think so. He loses every case we see him try, but it's not for a lack of ability so much as a lack of care. His strategies are good, if unorthodox for a prosecutor (because he's acting more like a defence attorney). He knows what he's doing in court, and although he definitely gets surprised/owned at times, for the most part he's pretty unflappable and fairly quick to recover. Basically, this meta by @theggning is great and I'm trying to restate it briefly, but go read the whole thing. I also think there's a great argument to be made for Mia being quite similar in court. Less cryptic shenanigans, but the same type of underlying confidence in her abilities/craft to back up her determination to protect her clients. Unfortunately, we've never seen her in a case outside of her rookie ones/being a ghost during brief points in Phoenix's trials, so I can't point to specifics as much.
ALL OF THIS TO SAY... as soon as I read your ask, I thought of Godot. I thought of Mia, too. I think that "a more confident Phoenix" would be a lot more similar to the both of them, than to anyone else. You could even say it's a natural extension of his arc, and that it's at least somewhat represented in AJ. In a funny way, you can also read that very same game's events as the reason why he doesn't seem as confident as you might expect him to be once he returns to law in Dual Destinies (his failure with the ace/getting disbarred leading to being somewhat unfamiliar in the courtroom after so long, the pressure of living up to his reputation, the heavy responsibility of protecting his clients again, etc. - you know, vs. the writers just wanting to go back to lawyer!Phoenix and not always thinking too much about his characterization during AJ and how that would develop when they did so). But certainly during AJ, especially from Apollo's outside perspective, I think we get a lot closer to the image of a supremely confident Phoenix Wright.
At least as far as I picture it! I don't think it would make too much difference in terms of him being unstoppable - since like you said, he pretty much already is. But there would be even fewer cracks for his opponents to poke at, and his mistakes wouldn't cut him down as much as they do in trilogy-era. He would inspire the same type of reactions as Godot did when he first appeared... a lot of people feeling intimidated and noticing just how well he seems to know his way around the courtroom, or people getting caught up in his pace and listening to him even sometimes outside their best interests. And, like Mia in the first game with Redd White, he'd scare villains enough to cause issues, probably. But at least he might (also like established lawyer Mia) actually be able to get his clients to pay him well, enough to buy fancy glass lamps if he wanted.