Posting my question here here rather than starting a new thread:
One of the reasons why there's a
superguide discussion thread stickied. :)
I'm gonna try address all your questions and curiosities.
If T-Mobile coverage is available for your area, and your usage is light enough to come in sufficiently under $20/month to make it worth it (especially since Ting is kinda expensive unless you're doing multi-line), Ting GSM does offer WiFi calling on their plans provided you have a T-Mobile handset that supports WiFi calling. I had used the function with great success in the past with a cheap Lumia 435, but this really isn't a good path to follow anymore for a variety of reasons. Similar can be said of Consumer Cellular using their own T-Mobile SIM card, but the same multi-line pricing caveats apply.
If you get excellent Verizon coverage in your home, consider looking into US Mobile or Red Pocket, both offer Verizon service and have some pretty good prices for "pay for what you need".
Boom's not terrible, but they've gone from launch with having plans on all four major networks to only Sprint and Verizon now (not a great vote of confidence), and their prices aren't terribly more exciting than US Mobile or Red Pocket on the cheaper end.
You could certainly get the phone fixed for cheaper, but it would require you doing it yourself, and built-in batteries can be a pretty big pain in the keester for a first time tinkerer to take on. Give the
teardown and replacement process at iFixit a look to see if you want to try. This is exactly why I recommend against phones without user replaceable batteries in general - but especially on proprietary networks, which pre-T-Mobile plan-based Republic Wireless was.
You should
theoretically be able to port your number out of Republic to any carrier, but the old VoIP Republic plans can sometimes be... troublesome... to port out of to some carriers. That's not entirely the carrier's fault, it's a consequence of how Republic used to set up their "wireless" service on Sprint.
As for where to find a good used Android phone, Ebay from sellers/dealers that specialize in cellphone refurbishing and have high reviews are usually a good bet, and Swappa can be a good choice as well. As for an Android phone I recommend these days, the Moto G4 Play XT1607 is a pretty solid device, all things considering. User replaceable battery, decent specs, good price new or used, usable on just about every network, aftermarket firmware upgrade path after Motorola abandons Android updates if desired. Just stay away from the ones that Amazon baked adware into.