Thanks, I definitely don’t get 5G with my occasional one bar of service, so this might be the solution.
For some reason, my phone’s menus say “Cellular Data” and “Cellular Data Options” instead of “Mobile Data” and “Mobile Data Options,” but we all know how Apple likes to rename its menu items. 
Update: leaving the phone out of airplane mode does seem to improve texting. The phone does use battery to look for an LTE signal, but nowhere near as much as it did when it was looking for 5G. The difference between looking for LTE and 5G is that with LTE, the phone can run all day on one charge.
I don’t know whether callers can reach me via Wi-Fi calling because I’ve now convinced everyone to call my landline instead. 