I think error is somewhat related to the wait time as it is being raised when the database cursor is trying to get the self._try_lock(). If you can then try to change your logic to:-

• Fetch the data from e-invoicing system first and then try to write the data at once instead of repeatedly. 
• If you don't want to change the logic then one way you can hack it to run the cron job every two to three minutes (or whatever suits you well considering the rate limit) and when it reaches the 30 limit you can just write it to DB and then CRON job will run again.