Actually, thinking about it a bit further, I am changing my logic. Instead of running a cron job for this, I'm just going to implement it into my frontend code.
So, when I "create" a domain, I'll have it create a record in the database equiv to what "postfix_domains" would have in it... In the same way, when I create a "list" I'll have it create the records in the database equiv to what "postfix_lmtp" would have in it...
With this adjusted logic, when I "delete a list" or "delete a domain" I can delete the appropriate database record as well... Ofcourse making sure to add some fail safe features into the code and/or database itself to prevent removing something that is still in use.
*** generally speaking removing all "transport_map" ("postfix_lmtp) entries for a list after deletion should be pretty safe, but there'll need to be a constraint and logic for ensuring the "relay_domains" ("postfix_domains") entry is not removed if it's being used by another list. ***
Ha, it was great chatting with myself on this... Got me thinking... Lol