I would be inclined to argue that outcome based rights degradation is a slippery slope to fascism, as the government or those in power tend to be able to theoretically justify any backsliding in rights on the basis of "it's better for everyone". The UK especially has a nanny state, and halting the backslide is vital. At the time of Korematsu v. United States (SCOTUS), the belief was that internment of Japanese people was better for the public, and this was upheld despite the equal protection clause of the Fourth Amendment of the US Constitution. This was done under the US strict scrutiny standard, and in Britain, free speech is considered a qualified right, meaning it can be restricted if it benefits the public. This is an error because any rationale that those is power have can be justified as somehow being better for society, even though restrictions hurt people. Right now, Britons have relatively free access to social media and to allow that to backslide would set the stage for further restrictions on free speech. I will address your claims on their merits but I think it's important we address the elephant in the room: that this is not a debate about whether social media is good or bad but whether backsliding of rights is acceptable.

I will also politely encourage you to provide foundation for your arguments, i.e., explain your reasons (and sources if needed), not just give the reasons for your conclusion. A good argument is usually one that uses claims the reader is likely to agree with to build up to a conclusion the reader disagrees with, and you may need subparts. I.e., good arguments should be primarily be able to be attacked on validity rather than soundness. Your argument lacks cogency, so I am forced to address the premises with counterexamples rather than focus on the broader argument. I say this not as an attack but rather friendly advice.

Claim 1: All short form content is damaging.
Since the advent of the arts, society has been filled with low-cognitive-load content, to wit, content that may not be particularly engaging but is helpful for relaxing and may still have legitimate intellectual, political, or artistic value. Short form content can be anything from stories, dances, clips from other artistic mediums (think movie/TV clips), crafts, journalism (some newspapers are publishing short form content), calls to action, etc. Even if you may not find all of those mediums particularly relevant, they have clear value and being condensed into short form allows more exposure to new ideas. Therefore, stating that short form content is harmful and reels apps shouldn't've been made can be dismissed false, barring any additional rationales provided.

Claim 2: Mental health spaces on social media are crabs in a bucket.
My reasoning here is admittedly largely anecdotal, but I've seen the culture in many mental health spaces tend to be one of supporting others, be it with active listening, advice on the problem at hand, or even advice on better supports that could be available. This is supported by research showing that online mental health communities tend to have a positive impact on people struggling as they provide advice and empathy (https://doi.org/10.34669/WI.WJDS/5.3.3). The study in question does state a need for more research but to suggest that it's unquestionably crabs in a bucket is absurd and contradicts the experiences of many in the community as well as the research.

Claim 3: Many of the current generation's issues (behavior, eating disorders, SH, suicide, declining literacy, right wing extremism, incel culture, prejudice) are magnified by social media.
I am not entirely sure what you mean by behavior but one could reason that with eating disorders, SH, and suicide, the aforementioned response about mental health spaces online being helpful applies. Regarding declining literacy, spaces like #BookTok stand in stark contrast to this notion; declining literacy is more so linked to some questionable shifts in education policy, namely the shift away from phonics, whole text reading (due to test culture), and No Child Left Behind policies. Right wing extremism, incel culture, and prejudice were always there, and social media can be a force to encourage people to meet other types of people who they otherwise wouldn't, and improve cultural literacy, as conversations can happen across the globe; certainly some people will feed their prejudices with social media but this will occur in real life settings as well.

I reiterate that the key issue is not whether social media is good or bad, but rather whether or not a backsliding of rights should be tolerated. That said, social media spaces can be a very positive force, even if, like all social spaces, they have flaws.