IMHO leaving the execute flag on TRUE while changing the index would be bad practice, as this could lead to inconsistent data. Process should always be: prepare command -> execute command -> read results -> prepare next command and so on. This also gives you a nice trigger flag && commands executed incr by 1.
You can however prepare multiple commands in a FIFO in your PLC to send them one after the other and store the results in a FIFO buffer and process them later and at a slower speed if your process in the PLC takes too long. But this is a unlikely scenario as you will mostly need the data from the RFID to control some process or some process has influence on the data you want to store on the RFID and you want to write specific data into a specific RFID tag.