So... apparently, the error was because the boost website didn't like the email address I was using. Probably because it had a ".fm" suffix(*) instead of a 3-letter suffix (.com/.org/etc...) When I switched to using a .com email address I was able to sign up.
Got the hotspot (a Alcatel Linkzone) soon after, and finally got around to setting it up a while after that. After verifying that the service worked, I switched the sim-card over to the hotspot that I had when I started this thread--a
GL-iNet GL-XE300--and it worked without any trouble. The administrator-screen on the GL-XE300 indicated that the network was T-Mobile.
Now, Boost doesn't really meet the criteria from my original post of being a high-quality company. I had to make a request in chat support to have Auto-pay turned off (it's possible I could have used their android app to do it, but it can't be done via their website), and I couldn't remove my credit-card info from their website, but I was able to replace it with the number from a low-balance giftcard I had sitting around.
So for around $13, I was able to test out my good router/hotspot, and acquire a (presumably carrier-locked-to-boost) spare router which can double as a USB power-pack for charging my phone. Eh...
(*) if you've seen that suffix before, you probably know who my email provider is.
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