Verizon is shutting their CDMA network down soon, so you will be required to have a phone that is at least VoLTE compatible. I don't think they are even allowing activation of CDMA only phones anymore.
Well...Verizon's plans to sunset 2G & 3G service in order to reuse those bands for 5G service could be put on hold for either of two reasons: 1.) if the T-Mobile/Sprint merger is blocked (meaning Sprint's midband spectrum remains out of play), and/or 2.) the FCC opens up new midband spectrum (CBRS) that Verizon reportedly has installed and ready to switch on to provide wider 5G service in urban areas. #2 just happened today, and could allow Verizon to keep the light on its legacy services well past the December 31 EOL date.
It should be noted that MVNOs like Red Pocket, that offer more than one MNO even when providing LTE service is because LTE isn't that old and established. Before the world-standard LTE for 4G was agreed-to, there were many small steps between 0G and 4G that followed one of two fundamentally different digital modulation schemes: TDMA and CDMA. To accommodate this technological bifurcation rather than set standards, the US has assigned roughly 100 different frequency bands to the various iterations, and very few phones (if any) support every last one. TracFone (a sister company to Straight Talk Wireless, mentioned by the OP) sent me 3 SIM cards with their kit, one for AT&T, one for Verizon and one for T-Mobile. Ting took my IMEI and sent me a "GSM" SIM based on the number. Google Fi sent me one SIM (and with eSIM let me provision my Pixel 3 through only their app) that works on T-Mobile, Sprint and US Cellular networks automatically. Red Pocket allows one to choose between the top 4.
I agree that an older premium phone is a good choice compared to what a new, inexpensive phone that might offer. I'd still want to talk to neighbors and use a tower finder (I use Cell Mapper) and verify the carrier(s) in play in the area. There's been a
lot of new low band work being done to extend the range of band 12,13 and 71 signals. Band 13 is used by "CDMA" carriers Sprint and Verizon, but 12 & 71 are T-Mobile. (I'm on T-Mobile via MVNOs, but don't see much low band in my urban area.) Red Pocket shows a red "CDMA" choice (for Verizon) and a yellow "CDMAS" that says "bring your old Sprint phone", so I presume that "CDMA" means Verizon in this case. I'd want to verify that's still the case before committing, though. Red Pocket also supports both "GSM" carriers. With 4 MNOs to choose from, I'd want to get the best one!