Selectel (official recommendation - oldest non-America Movil provider, has roaming) or
Puppy Wireless (unofficial - Kitty Wireless owned, but still new and no roaming). The other two options (Tracfone and Page Plus) are owned by the same corporate overlords now, and you know my feelings on Carlos Slim's offerings.
Going 6s may be a PITA for activation with a Verizon MVNO, though, as the IMEI/ESN
MUST be in the Verizon database for activation, so no buying the unlocked T-Mobile handset and bringing it over, and buying the Verizon handset requires activating a line and account for at least a month with Verizon postpaid first. Get a used Verizon branded 6 instead with a clean ESN/IMEI that's not under contract or buy a 6 from Selectel directly.
As for plans, Selectel's cheapest LTE plan is their $100/year for 2000 minutes and 2000 SMS messages with 2.5MB of data for MMS, and no mobile data without buying $10 Flex Cards for data billed at 5¢/MB. The annual plan is a use it or lose it, but the Flex Card balance doesn't expire. After the first year, so long as you have Flex Card balance active on the account, you can technically (for the time) get PAYGO going for 5¢/minute/SMS/MB - but the Flex Cards are technically designed to be paired only with an ongoing monthly or annual plan, and not meant to be used alone. It is a billing fluke that others have used, however, with great success in the past.
Puppy's best priced LTE plan is $15/month for 250 minutes, plus 250 SMS messages, plus 100MB of data, and as this is a monthly plan, there's no balance rollover. Strictly use it or lose it, and over-use it, you're going to need to cover overages. There's also a $10/month plan, but it's only 60 minutes, 60 SMS and 2.5MB of data. Puppy does not offer annual plans.
Unfortunately, you're wanting to play with the most expensive phone on the most expensive network. Them's the prices, no matter how little you use. Selectel's probably your best bet between the annual plan and $10 Flex Cards for a rough monthly cost of $9.34 (that's assuming your data use doesn't spike with the new phone). You can potentially get marginally cheaper "per month" as has been pointed out, but their service level and account management is something that leaves me incapable of saying nice things, and the actual cost per minute/message/MB isn't that hot. That's also assuming you can even activate a Verizon iPhone 6 on their Android 3x plan on the Verizon network. And I won't even get into the possible nightmare scenarios with them and ESN locking/blacklisting...