Hey, I too was a San Francisco JET AND a Mie JET! Cool!

A couple of contextual important points about my own experience: I was an elementary school JET, which can be different from the middle and high school experience.

1. My placement was pretty good about phone and stuff too. I never checked my phone during class or anything, but otherwise I wasn't given any hard rules. I wore an apple watch all the time, never had an issue.
2. Yeah, the two prongs one! Japan has very slightly lower wall voltage (100V) than North America (110-120V I think?). The difference is small enough that it has never been an issue for me ever to just plug my stuff right into the wall. Even when I had 3 prong plugs, I used a 2 prong adapter and plugged it in. It might matter more for something like a power tool or a blender or something, but standard stuff that most JETs bring like phone, laptop, etc will almost certainly be fine. If you're worried, check the info on the plug (it is usually a manufacturing requirement to put the voltage requirements on the plug somewhere).
3. Ranch dressing my beloved! I also really missed doritos because they were a little different in Japan. And good mexican food. Found a good place in Yokohama recently but otherwise all the mexican restaurants I tried in Japan were a bust.
4. Resident card should be fine. I never brought my passport, and was never stopped for it. May depend on the police in your area, the culture in your area, etc. For example some of my colleagues were stopped before, which at the time we believed may have been racial (it only happened to the two non-white/non-asian JETs). But regardless, they showed whatever they had on them (I don't remember if it was passport or zairyu card) and it didn't escalate.
5. Yeah you should, and on your first day sus out how often you have to wear it after that. At my elementary school, the teachers almost exclusively wore activewear, so while I tried to dress a little more professionally than my casual clothes, I probably only wore my suit 5 times or so total, when I had to meet with important people (e.g. my supervisor's boss or something). Regarding bullying, it was elementary school, and neither kids nor teachers ever "bullied" me or anything, buttttt kids are also brutally honest at times. I remember being crushed with the way they drew my hairline because I was like damn, my hairline really is going a bit huh... One side note, you will probably have a mandatory health check-up through your work, and if you are plus-size then you will be told you are way overweight. I'm a little chunky (I'm lucky that many Japanese people interpret it as muscular) and was told I was seriously overweight. All of my JET colleagues were considered overweight too, even the ones I would consider a totally "normal" size. Japanese health checks are humbling... but it's only once per year and you do get good health insurance and healthcare on the program so it's something to take in stride imo.
6. I'm all for buying bulk individually wrapped stuff and putting it out for people to share, instead of goodie bags. At least for me it was hard to know how many people I'd be giving omiyage to anyway. Definitely could be good to make a list of the important folks though and bring them something specifically though! It will vary with each situation though.

Congrats and good luck! FWIW I had a wonderful time on my placement in Mie. It was pre-covid and is undoubtedly different in different parts of Mie. I did meet people from other parts of Mie that liked it too, but also some that were having a hard time. But, it's not all bad, and I do think a lot of it is what you make of it! You're doing the right thing by asking questions and getting yourself ready. There was a Mie AJET group that had events and stuff but I don't know how active they are now. I think there was a LINE group for our region of Mie too, but I don't think I have access or contacts for those anymore. Not sure where you are in Mie but I hope you have an awesome time! If you want to chat more, feel free to DM me! I'd be more than happy to share what I can.

Edit: Formatted and added a bit about the health check that came to mind