This is on Mint Mobile, a budget MVNO now owned by T-Mobile. I pay $20/month for 15GB data.
I think a lot of people are attributing speeds of 200-700Mbps to mm wave when it's actually pretty ordinary speeds for 2.5-5GHz coverage today now that carriers have decent capacity there. If you're getting below 144Mbps, you're in the bottom 25% of T-Mobile's network experience. Over the past 6 years, speeds have gotten a ton faster, mostly from 2.5-5GHz spectrum. Verizon is the only carrier that put much effort into mm wave and it still covers basically nothing.
Ignoring latency, you're likely to get speeds similar to wifi from today's mobile networks without mm wave - unless you have a really great home internet connection and a good wifi setup. I have a MacBook Pro, but it only has wifi 6 and I can get 700-800Mbps out of my wifi, but I'm not getting 2Gbps from it.
How is it a better deal to spend more money for essentially the same thing?
Imagine you're buying food that will go bad tomorrow. You can't keep it. You can't share it. You can buy enough food to fill you up today for $5, or enough food to fill you up 50x for $20. You will still eat that one unit of food, the other 49 units will just go to waste. Which do you buy?
For all the time I've had Mint, I've saved nearly $3,000 compared to your $50/mo plan. I've never felt the urge to really need more data personally, and it's worked well for me on my international travels around North America. If the outcome was the same, wouldn't you rather keep $3k in your pocket?
I have it in my home, but I live in the center of a major city. It just speedtested at 476.64 Mbps down, though I think I've seen 650+ in the past, but I might be mis-remembering that.
However, I only get 5G mmwave (indicated as "5GuW" on the phone) on my work phone.
The carrier I use for my personal phone charges an extra $20/month for 5G, and I don't care that much, so I leave it at 4G LTE.
Edit: Just re-tested and got 767 Mbps.
C-Band spectrum also makes sense given a speed test of 400-800Mbps. That's fast, but not hugely faster than what you should expect with really good coverage today. T-Mobile experiences speeds above 560Mbps 25% of the time. Verizon's 5GUW (which is mostly C-Band) experiences speeds above 620Mbps 25% of the time. You ran two speed tests and got 477Mbps and 767Mbps. One of those is pretty average for C-Band and one of those is quite good for C-Band (25% faster than the 75th percentile), but nothing too out of the ordinary.
Those are speeds that are relatively common when carriers have around 150MHz of 2.5-5GHz spectrum deployed in your area and you have good signal and not too much congestion.
mm wave does exist, but the coverage is often a block or two with indoor coverage being really spotty. C-band coverage will be measured in square miles. C-Band won't have the coverage that lower frequencies have, but it's a lot more practical to deploy than mm wave. mm wave shines when you have a small, high-value area where you want to deploy 1,000MHz+ of capacity. Carriers might have 50MHz of capacity around 800MHz, 90MHz of capacity around 1.9GHz, 150MHz of capacity around 4GHz, and 1,000MHz of capacity around 30GHz. That really high frequency mm wave won't cover a large area, but there's a lot of it. If you're looking to cover a sports stadium, it makes sense. If you're looking to cover people's homes (even in major cities), C-Band is going to get you really good speeds with coverage that will be a lot more usable.
It's possible that you're getting mm wave, but those speeds are pretty normal for good C-Band coverage. The first test is pretty average and the second test is quite good, but nothing extraordinary.
Chinese cities, especially at train stations and the surrounding area get really impressive flows of people every day. But even in China I would expect that while they might have a lot more 5G SA when in the US, that the radios are actually mid band and not millimeter wave for most coverage. You still get higher capacity and low latency, but most people probably don't notice the low latency, except maybe Chinese autonomous taxis faking it till they make it.