SoftwareSerial doesn’t work well at speeds higher then 9600, especially on a device like an Uno.
However, you can’t just change this value in your sketch. You have to re-configure the ESP-01 to work at the lower baud rate, using AT commands.
TBH, you’d be better-off throwing the UNO/ESP-01 in the bin and using a NodeMCU or similar, as this would also fix your memory issues…