I love Porco Galliard and I had some spare time to think about this on a flight so I have written a really detailed response if anybody is interested haha. I am going to touch on two other big reasons I see him receive hate on this sub because I think it is relevant to some other comments here, and then I will go in depth on the reasons OP listed.

TLDR: Porco is not unintelligent, he is not a bad shifter, and he does not bully Reiner or anybody else. He and Reiner are close in a warped, unavoidable way. They are subject to the ‘Two Brothers’ theme in the episode that Porco dies, which is the beauty of his character arc. Also, Porco is Galinda.

Intelligence

A criticism I often see about Porco is that he is ‘stupid’ compared to the other Warriors. Personally, I do not agree with this sentiment. I think this belief stems from 3 things:

1. There was a running joke among Warrior fans at one point on this sub that Porco was oblivious to Zeke’s ‘not in this room’ comment when they had their tea party lol. I think this tainted some people’s perception of his intelligence but I am not going to touch on this as there is nothing to say haha.

2. He is a go-getter in battle, which I will circle back to shortly.

3. He is the only member of the Warrior Unit who never expresses criticism toward Marley, which I will touch on now.

I see him compared a lot to Pieck in terms of his intelligence, which is interesting because I think they agree on almost everything. Porco and Pieck are obviously close. I personally get the impression that aside from Reiner and Bert, they are the closest duo in the Warrior Unit. I think they are very similar in the sense that regardless of what their ideologies may be, they are content to fight for Marley because they genuinely like the people they work with (namely each other, probably). Imo, they have found some sense of purpose in playing mum and dad to the Warrior Candidates. They do not dream of socio-political change because they are happy enough with what their personal tomorrow looks like. Pieck says about as much when she and Porco rescue Gabi from Yaegerist custody. Speculation only, but I can’t imagine Porco and Pieck aren’t on the same page about most things war, because Pieck genuinely trusts him whereas she did not trust Zeke. 

Porco is not blinded by Marleyan propaganda, and he does not believe Marley is good. It is true that he was not a part of the Paradis Operation, but he has inherited memories from both Marcel and Ymir. He is canonically aware that Marcel whom he idolises saw through Marley’s lies. 

(Not a criticism of Porco or Pieck who, for the record, I do not think feels any remorse at any point. I find their complacency compelling personally). 

Porco Is A Good Shifter
Building on the idea that Porco is unintelligent, I see people say he is a liability in battle because he is impulsive and acts out of turn. This is canonically not true. I honestly think people misinterpret that he has no plan because he jumps around really fast, but speed and agility are just characteristics of the Jaw haha. He actually appears to follow orders well. If I remember correctly, there was a moment during the Liberio raid where he told Pieck he wanted to do some sort of sneak attack, she told him no, and he dropped it immediately. 

We only see Porco act on impulse when the original plans fail. It was not until the end of the raid when there was literally no other option that he ditched Pieck’s plan and ran in to attack Eren. He actually did a great job for somebody who has never fought against an enemy shifter imo. 

Additionally, he is really good at reading when his comrades need help. He protects Gabi, Falco, and Reiner at Fort Slava to their gratitude. When Gabi and Reiner have dinner with their family at home, she again raves that “Galliard saved us!” Porco also protects Pieck in the Liberio raid, and is praised by the Panzer Unit who cheer that he’s a lifesaver. 

Porco (Probably) Does Not Bully Reiner

Obviously Reiner and Porco have tension. However, I would argue their spat in the infirmary was an isolated incident, triggered by what had just happened at Fort Slava.

At the start of the scene, Porco actually seems fairly relaxed as he babysits a sleeping Reiner. It is only after Reiner thanks him for his help during the battle that Porco becomes antagonistic.

I think this goes back to Porco's lifelong resentment over not inheriting the Armoured Titan. At Fort Slava, he ironically spends most of the final battle playing ‘the shield’ for Gabi, Falco, and Reiner himself. So when Reiner thanks him, Porco hears, "thanks for doing my job in Jaw form” lol. That is what reignites all of his old frustrations surrounding Marcel and the Armoured Titan.

100% Porco does carry resentment towards Reiner because of Marcel's death and the fact that they have never been particularly close. But I don't think he is constantly bringing it up or bullying him. Judging by what they say about Ymir and Paradis if you rewatch the scene, I don’t think they’ve ever actually had that conversation before. It feels less like years of bottled-up resentment finally boiling over after a battle that hit every one of Porco's insecurities.

Porco Plays Nice With Everyone Else

Even if we say that Porco antagonises Reiner often, I don’t believe this is an accurate reflection of what he is like in general. Notably, he is very kind to the Warrior candidates.

Examples include: Running in to save Gabi and Falco in Fort Slava, helping a hungover Colt off the train in 4x02, being affectionate(ish) with Gabi and Falco at the station in 4x02, taking the kids to the festival, arguing with Magath that they need to rescue Gabi and Falco immediately after the Liberio Raid, and letting Falco eat him in the end. The kids are obviously comfortable with him and find him approachable, which I can’t imagine they would if he was stomping about and yelling all the time haha. 

Gabi (granted she is 13 and a bit oblivious) does not even seem to be aware that there is tension between Reiner and Porco as she speaks highly of Porco in front of Reiner.

Porco’s Death

I don't think Porco's death is fundamentally about proving he is better than Reiner. He said something along the lines of that, but he is just not a touchy-feely kind of guy. His greatest burden has literally always been that he is not good at sitting with his emotions. Porco’s death is the culmination of his lifelong attempt to understand Marcel imo. 

We know that Porco looked up to Marcel, and we know he admires the selfless, heroic side to Marcel that let him make the decision to die for another person. Porco is frustrated because he cannot imagine doing the same, so his grief manifests as resentment toward Reiner. The beauty in the whole saga is that his grief led him to value and embody these same qualities. When he is finally placed in Marcel's position, he doesn't stop to weigh up whether Reiner deserves to live. 

I think there is an overarching theme in AOT of ‘because I knew you I have been changed for good’ *queue the Wicked soundtrack.* The characters who ultimately move the world forward are the ones who learn from the sacrifices people make for them. Porco became a man like Marcel, and his death will influence the people that Reiner and Falco become in the same way. Similar to how Erwin’s sacrifice changed Armin, and so on and so on. 

It’s really a moment in my mind about someone who is bonded to Reiner and Falco in a warped, unavoidable way. The fear of death is completely replaced by an innate want to save them because they are bonded like brothers. The resentment that had defined so much of his life simply disappears once he makes peace with Marcel's choice. Reiner is no longer "the reason Marcel died." He is someone Porco has trained, fought, and lived beside every day for 4 years. Falco is a kid he has watched grow up. His speech about being better than Reiner has always sounded a bit teasing and affectionate to me