If the phones are CDMA only, pre-LTE SIM card with Verizon, you're on borrowed time anyway. The 1xRTT CDMA network is getting shut down next year. You need to upgrade phones. You upgrade phones, that'll theoretically fix the MMS issue (MMS also needs a data plan). There's no such thing as feature phones anymore with phones that require VoLTE (Voice over LTE network) support, as even the most basic phones are running ridiculous processors and using stripped down versions of Android and KaiOS (Firefox OS) to make this happen, so you're still going to be stuck with convoluted and overly complicated user interfaces and terrible battery life, even if the form factor still looks like a feature phone with buttons. You're also not going to get much of a selection as Verizon currently only sells one LTE "feature phone", the Kyocera Cadence. This said, you may get improved coverage with a VoLTE capable handset. 4G LTE tower density with Verizon isn't as important as the older CDMA days. One tower in the 700MHz spectrum can do line-of-sight up to about 40 miles. You may not need a signal booster.
Regarding VoIP with a WISP? The problem isn't bandwidth. You can do VoIP successfully over the bandwidth available with a 20 year old 56kbps dial-up modem. The problem is network latency, jitter and packet loss, or signal delay between locations and loss of transmitted real time sensitive data. WISPs and wireless internet really don't lend themselves well to VoIP. If that's your only option for broadband, don't even bother trying that path. You need something wired with infrastructure in decent shape. This also means that using WiFi calling support with a supported Verizon handset or a femtocell is off the table, as the same shortfalls will impact call quality there as well as VoIP for the exact same reasons. There are passive directional yagi antenna solutions for boosting signal strength with Verizon service and your nearby towers, but they're imperfect... and given the network changes coming to Verizon as the old CDMA spectrum and towers gets refarmed in a few months anyway, may be a bit of a pointless investment.
As for Tracfone, Tracfone sells cheap, flimsy handsets. They also sell plans and phones on multiple networks, so unless you know for certain that it was using Verizon and also had VoLTE support, you have no guarantee that it even remotely approaches realistic modern coverage expectations with Verizon. If you want to switch to a Verizon MVNO, ideally search one out that's not owned by TracFone. Unfortunately, there's not a lot of Verizon MVNOs out there outside of various TracFone shell brands (Total, Page Plus), and the prices make Verizon Prepaid attractive unless you're a light user. If you want a feature phone with a Verizon MVNO, you already know the only option available with Verizon VoLTE support. If you can't find the thing used with a clean IMEI/ESN, you're going to have to do contractual obligation time on Verizon postpaid or prepaid first before porting a new one over.
You're kind of stuck between a rock and a hard place unless a competitor's coverage has drastically improved in your region, or if getting a proper Verizon VoLTE handset improves coverage. Sorry.