I often am on WiFi with very little service and message+ tries to send it over service and if fails. I then have to turn on airplane mode and wifi to get it to send over the WiFi network. Is there a way to make this just happen automatically?
It is supposed to do so automatically. What phone are you using?
Galaxy S6
But would that make it so if I don't have wifi the messages won't sync at all?
Please READ "Sync Over Wi-Fi only when connected to Wi-Fi" does not imply it won't work when wi-fi is not available. If you're not on wi-fi it won't try to sync over it.