The maximum transmission unit (MTU) for TCP is 1500 bytes. That’s why your transmissions over that size are failing. The commands TinyGSM uses for the SIM7080G are too low-level to break it up; you’ll need to do that yourself or submit a PR for TinyGSM. The XBee commands are higher level; it does the break-up for you.
My first guess for the modem stopping responding would be that you changed the baud rate to something too fast for the Mayfly and the communication because too unstable, so it stopped responding. I can’t think of any other commands you might have read online that would result in something similar, but I’ve messed up more than one type of modem playing with the AT commands. Our field failure rate of the SIM7080G is lower than for the XBees. Shannon could give you a better guess of the number of field failures than I can.