My pleasure. I don't get into moodle.org as often these days but I do enjoy the certificate module and the ways it will automate if you think outside the box. Another thing you can do is issue the key in the cert it's self if you don't have to do grading.
The flow would be- complete course 1 and the key to course 2 is in the custom text field of the certificate, complete course 2 and the key to 3 is there and so on. The individual classes can be Plan Jane certificates then do a nice one with a custom seal watermark signature and text for the final one