The final point I still haven't quite gotten over is the squick of the "poor person carriers." The only reason someone would use Cricket or Boost or one of the "Pay ahead of time" carriers, in my mental map of the world, is because they can't afford a proper carrier, or can't get past the credit check. Theoretically Project Fi is one of those, so I shouldn't mind as much, but... eeeh.
I know I'm late to the party, Synoyk, but I thought I'd especially address this one point for you. In the global mobile phone industry, postpaid carrier plans in this country are the abnormality, not the standard. Over 85% of the world's mobile plans are prepaid. We didn't even start to see any downward trends in postpaid pricing in this country until wholesale MVNOs finally made a dent and started eroding the big four's profit margins instead of bolstering them around 2013 or so... it's bonkers that it took wholesalers reselling airtime in smaller packages and providing their own customer support to actually drive down mobile prices in this country, but that's deregulation for you.
The only real benefit with "postpaid" plans are the addition of off-network roaming. Consumer Cellular and Ting being the big two aftermarket MVNOs that do that, and neither of them do credit checks. So relax, and ignore the idiotic old bias in this country that prepaid is for deadbeats and drug dealers. It's more cultural arrogance and discrimination than anything... after all, there's a deep seated cultural hatred for the poor, living within your means, and only paying for what you need in this country.
Which brings us to the second part... AT&T's antitrust history, Cricket Wireless, and the exploitation of the poor. You've hinted over the years at matters of faith, ministry, and outreach. Approach this matter from the lens of your faith and read Cricket's terms of service and their price schedule. Cricket exploits the most financially vulnerable in this country with their various maintenance fees in ways that no other carrier has. Don't just take my word for it, though... research yourself... but rest assured that Cricket charges for things that no other prepaid carrier brand or MVNO does, and the way they've set those fees up, they actively exploit the most financially vulnerable in this society. They're blatant poor taxes. And that's ignoring the other problems with Cricket such as the undercutting of their wholesale pricing with their end users.
Remember, AT&T didn't give a toss about prepaid and lower monthly bills in this country until their own wholesale customers peeled off enough angry and overcharged postpaid customers to make a dent in their ridiculous profit margins. Now, because there's no regulation preserving fair wholesale pricing and network access, AT&T (along with Sprint and T-Mobile) are actively trying to undercut the third party MVNOs that forced them to be serious about offering prepaid phone service in this country and to reclaim higher profit margins by killing off their competition and bring the customers back in-house. What happens when their real competition disappears? Given the recent FCC policy changes regarding the Lifeline program, wire network infrastructure maintenance, broadcast television, monopoly media holdings in service areas and all the other skullduggery on top of the network neutrality repeal, I suspect we'll get those answers far sooner than later.
As for the iPhone/AT&T MMS issue, it's been brought up already by others, but Apple deliberately breaks MMS functionality for any AT&T MVNO that isn't on their approved provider list. Just another obvious reminder that you don't really own smartphones, doubly so for Apple devices. Now they're introducing embedded SIM cards, restricting your carrier choices even further. The Cupertino noose is tightening.
As for yourself, I know you've settled on AT&T prepaid. Here's some additional future food for thought, though. Even accommodating your existing biases and existing hardware and usage patterns, going Consumer Cellular using their AT&T SIM card may still be cheaper for you in the long run. $20+tax will cover all your needs most months. You can set their data plan to 250MB (which you could probably easily hit on the light months with some additional tweaking since you're already in the ballpark at 300MB), and if you need more, it'll just pop you up to the next tier of service and notify you before doing so. One line with your call load, even with 1GB of data is still only $25+tax a month. There's higher tiers available if needed, and adding a second line allows you to pool usage and potentially save more. You'll also get the full AT&T postpaid carrier footprint as you'll still have off-network roaming access for those prices - something that you will lose with AT&T Prepaid. Just something to consider longer term. You still have room to save another $5/month on average, you'll get the same network coverage you had, the customer support will be more pleasant, your postpaid itch will be scratched, you shouldn't have any problems tethering, and it'll work with your iPhone as an approved carrier thus keeping MMS functionality.
Anyway, enough out of me. Just figured I'd add some points for consideration.