Usually, I dont use the tx_interval variable to declare when it has to send a lora message and do it somewhat external. I have a timer who stops the program for x minutes (either sleep/stop/standby mode). In stop-mode, all clocks except RTC (whose only task is to give an interrupt when stop mode is finished) are stopped. But in stop-mode, it still works and it still does send a message out to the world, while in standby it does not. In neither modes, the internal clock should provide a tick-interrupt, since every interrupt would wake it up from any sleep-stop-mode (except standby-mode) and has to be disabled.