Polytonic,
Your statement of:
"Functionally speaking, in Canada/Mexico you'll get LTE data speeds if you're on the Magenta or Magenta Max plans, and then unlimited 2G (128kbps) everywhere else in the world. You can pay a bit more to upgrade (one or more lines) to Magenta Max to double the international data rate."
Is not too accurate since I am on a TM ONE Plan and it has the ONE Plus Promo included for 256kbps international data roaming on all 12 of my lines. If I'm looking for convenience I would seriously consider an after market data esim like Airalo instead of the high priced Google Fi. It is just as easy to use and set up with IMS service with a dual sim phone and keeps my main voice line active while roaming for free calls to and from the US. I can get 5Gb all over Europe @ $20 good for 30 days. Many on this forum have used and recommend Airalo, especially compared to Fi. It has been discussed in the past few pages. I'm pretty surprised you would still consider Fi as a viable alternative. Even T-Mobiles 30 day Global Plus Plan gives 3 times the data (15Gb) and unlimited free calling with 5Gb of tether for $50.