Basically, the high-level answer is that you probably aren't going to find exactly what you want. I've had all four major providers in recent years. Pretty much anything else you get is going to be a MVNO reselling one of the big four. (Google Fi is different in that it's reselling both Sprint and T-mobile.) Here is my experience, primarily in California, but also in several places I travel to on the East Coast:
* Sprint can be borderline useless. The coverage was so bad that I missed many phone calls. And the data coverage completely fails in shocking places, like, say downtown San Francisco. I used a Galaxy Nexus on Sprint for 18 months and was miserable.
* T-mobile has the best plans. You can get unlimited high-speed data pretty easily (which is not the same as what they call unlimited data) and use it for tethering (google "tether_dun_required"). Moreover, unlike other carriers, they are transparent about their throttling policies. If you are in the top 3% of data users (over 21GB/month), you get deprioritized, but not throttled.
* Verizon has the best coverage, but you will pay through the nose for the 100GB/month you intend to use. Also, for iPhone it might matter less, but Verizon seems to be the most aggressive about "customizing" their Android phones with bloatware and value-reducing software. For example, they make it hard to tether. Even on my rooted Nexus 6 I can't figure out how to tether with the stock android distro, because it installed some kind of crap when I inserted the Verizon sim. Note that you *can* buy grandfathered unlimited high-speed data plans on ebay, but this is going to be super expensive for you, and probably not work if you can't do post-paid. Also, it doesn't seem like a good investment for the future, because they can take your plan away. (I had an unlimited data plan on 3G, and they wouldn't give me a SIM card for 4G unless I changed to a metered plan.)
* AT&T seems like a not great compromise between T-mobile and Verizon. The coverage is better than T-mobile, but nowhere near as universal as Verizon. Particularly in California, there are many places AT&T does not work. AT&T also doesn't have great data plans, and customizes their phones more than T-mobile. A few years ago, AT&T's voice quality was really bad. I dropped them after doing an experiment where swapping the SIM card for a T-mobile one into the same phone made call quality noticeably better.
So after many years of switching between carriers and finding none is perfect, I now have two phones. I use a T-mobile phone for day-to-day stuff and (of course) when traveling internationally, and a Verizon phone for when I'm in areas with no coverage. I also have a Verizon hotspot, which I use for data. And when the Verizon hotspot stops working well because I'm surrounded by too many other data hogs, I switch over to tethering with T-mobile (whose network seems to be less loaded in the places I travel).
So my high-level message to someone who wants to come to the US 3-4 times a year and use 100GB of data in rural areas but not pay too much? Lower your expectations, as you will have to compromise on something.