I have never heard of google Fi so I spent some time looking it up. Fascinating system and might be a cheap alternative to try on a European trip. There are not as many anti-Fi reviews as enthusiastically evangelical reviewers so a few hours on the Net could be enlightening. Of course, the webs are crowded with loud and angry voices, and, if you read between the lines, you may see many of the negative experiences are based on user errors and bollixed payment setups. Several of the high tech review sites are quick to point out the user base for google Fi tends to be tech-savvy and relies mostly on self-support forums. Customer support is often described as poor. Fi is a mobile virtual network operator (MVNO) that piggybacks on existing cellular networks in the States and 200 countries. Basically, Fi hops on and off whatever system is available wherever the phone is located.
One can apparently bring one's own phone but loading up google Fi on an iPhone appears to remain a beta product, requiring running an Android-based app on top of the iOS in addition to replacing the SIM. Easier, naturally, to just buy an Android phone (Pixel3 <$400, Pixel 4XL <$900, Moto G7 <$300, used Android phones are easily found).
Back of the envelope (and I am not a user with ANY experience), with two minimum phones and unlimited coverages for two would be roughly $1000 for two months, far less, naturally, if you don't need the hardware, about $250, less again if you don't need unlimited data, maybe $100-$150 fo pay-as-you-go. According to people who are supposed to know this stuff, Fi is not the least expensive option, not the easiest to set up and use, nor the best option for high data users. You gotta do the research.
Plenty of caveats and you must have an active account and data connections established and tested in the States long before connecting to the international services. If you travel internationally often, and have the cranial capacity to learn two phones, having a dedicated Android phone on google Fi might be a viable economic alternative. Just do't try to get it all together the afternoon before your plane leaves.