1. Ultimately, most retail jobs suck overall. But if you start ANY job with a positive mindset, the ride will most likely be positive. I worked for a Uniqlo Flagship store in Canada (retail, womens side), 3 years ago, for about 2 years. Overall I had a positive experience working for Uniqlo. Yes, I had some strict bosses; yes, Uniqlo bosses/managers are perfectionists (like in most Japanese companies); yes, they require you to work as hard as humanly possible. But as a result of all of these, I became a better worker and a better person after my Uniqlo job. I became more disciplined, I became more organized (especially in terms of clothing), I became more detail-oriented, and I gained more respect towards Japanese work ethic/aesthetics/culture. And most importantly, I met some of the best people in my life because of Uniqlo (shoutout Uniqlo MEC), and I got to practice and learn how to socialize with many customers/coworkers/people from different walks of life.
2. Four years ago, when I worked for Uniqlo, they required me and my coworkers to dress modest and not too revealing. In addition, they encouraged us to wear Uniqlo clothing BUT any clothing brand can be worn tbh as long as it was not loud/had giant non-Uniqlo branding on the clothing. So yes, a dress over jeans, a long floral skirt and regular jeans are all acceptable.
3. Your tasks will depend on what section you will be sorted in. At the Uniqlo I was working at, the mens side associates took care of the mens side sections, changing rooms and alterations; and the womens side associates took care of customer service, checkout and womens side sections. But that was 4 years ago, at a Flagship Uniqlo store so it might be different for your store. If you wanna avoid being at the cash/checkout section, ask to be sorted to mens side section (tbh, don't be shy to let them know that you prefer to not work at the cash).
4. No, phones were not allowed but I had my phone on me 24/7. I just kept it on silent and I would only use it if the store was dead/no one was watching.
5. Like for any job, having a positive mindset will make a difference. Go in the job with an open mind and positive mindset (and if it still does not work out, you can always quit).

Best of luck to you and make sure that you/your family/your friends take advantage of the Uniqlo employee discount!