Same advice for you as I give everyone with iPhones coming off of Verizon towers (and sometimes lately also considering Google Fi). Don't throw good money after bad. You wasted the money on the iPhones, use them up and wear them out.
The Verizon iPhone handsets from the 5 on are magic ticket handsets that you can use with any carrier and are unlocked. That said, their antenna design is notoriously sub-par for a smartphone. (This isn't a Verizon branded issue, this is an every Apple handset made issue.)
Verizon MVNOs are some of the most expensive available.
The iPhone is unfortunately one of the most expensive mobile phones to get service for because Apple only has a limited "blessed" provider list and vendor lock-in designed to surgically extract as much money from you as possible, where they cripple features with many carriers, and non-approved AT&T MVNOs are the worst of the lot as you'll lose MMS functionality, which makes iMessage damn near worthless frequently with non-Apple users with group, oversized and multimedia messaging (people use MMS a lot more than they realize).
AT&T with roaming is the only network that's nearly analogous to coverage to Verizon with roaming (though you're in a dense urban environment, nearly everyone should work - but we're assuaging fears here to make it easier to switch), which really only gives you four choices: AT&T's "boutique" in-house MVNO brand that has financially exploitative terms of service and poor taxes, along with customer service that makes AT&T under-contract postpaid customer service look pleasant and data pricing that screams for mobile industry regulation and equal wholesale pricing access; Carlos Slim's MVNOs who no longer offer roaming with their plans, have punitive terms of service, and lousy customer support; Consumer Cellular which offers either AT&T or T-Mobile SIM cards and is blessed by Apple and on the approved guide list of providers, but is really only a deal for multi-line accounts; and Selectel which is the only remaining Verizon MVNO with roaming, is the only remaining recommended Verizon MVNO in the guide at all, and is expensive.
That said, Ting CDMA does offer roaming off Sprint onto Verizon and USCC and Verzion iPhones should activate on Ting CDMA, but then you're dealing with Sprint as your primary carrier which means Ting GSM would be a better choice... but again, like Consumer Cellular, Ting is cheaper for multi-line households, and honestly Consumer Cellular is a better deal for most people due to the difference in bucket usage structuring, especially for people who desperately need to go on a data diet but refuse to do so. iPhones should also work with all T-Mobile MVNOs including US Mobile (unlike with AT&T MVNOs) as the MMS settings screen isn't locked out (yet), but T-Mobile without roaming can be difficult for certain regions of the country (yours shouldn't be one of them unless you frequently travel to West Virginia or rural Virginia - but check maps anyway).
As for Google Fi specifically, when you factor the added electronic waste generation and demand along with the actual cost to switch versus staying put and going with another provider on your current phones, the math over the course of a couple three years doesn't frequently make sense to make the switch. Also, Fi has a bit of a quirky, schizophrenic quality to it when trying to connect to mobile networks and WiFi locations, which can lead to uneven call quality and potentially dropped calls due to too many mobile networks to connect to in some areas and their reliance on UMA network calling. UMA as a technology is fine, and having access to multiple GSM networks for roaming is nice, but execution on the combination and the signal algorithms for Fi leaves a little to be desired under real-world usage.
tl;dr: Keep your ridiculously expensive phones, use them up and wear them out, switch to Consumer Cellular and get their AT&T SIM cards, and ignore Google Fi.