You should definitely sleep for at lease 5 seconds between each run - I found that ZAP takes a little while to shutdown in the background and sometimes previous runs interfered with the following ones.
Did you mean to include the '-j' parameter? Thats not actually supported for the API scan.
Also you will just keep on overwriting the report.html file each time so you will only end up with the last one.
It might be worth trying to adapt the Mass Baseline scripts :)
Cheers,
Simon