That script can be run anytime you desire and it will read the data from the hard drives and send it to you in an email. I run a script similar to this myself at 10AM each morning this way I know that if a SMART Long test was scheduled for the day, the test "should" be completed by the time the script runs. Sometimes the long test isn't completed because I had disk activity which does slow the test down, but then I get a report on how far it's tested and I'll have to wait until the next day to get a complete report.
My point is that I think you are trying to do it at a good time. If you are sending the report to an email address that you check after 8PM Monday night then that is fine, but I would send it Tuesday morning at maybe 6AM (just before you would normally check your email). In other words, the results can wait until you are ready for them.
Also, if you are testing all the drives at once for an Extended test, that will slow down NAS operations some but definitely slow down the SMART test. If this is in a work environment and you expect people to use the NAS for real work, maybe test 2 drives a day it mitigate any slowness. If this is a home system and it's not a big deal, then what you have going on is fine.
