It sounds you are storing your schedule of tasks in the database. No problem. For every different type of task (eg. sending newsletter, save reports) create a service, which does the task. Then add to this services a tag (like twig.extension, but your own), so you have something like a TaskChain, which knows all the tasks.
For executing create a console command, which retrieves the schedule from the database, loads the TaskChain, and executes the tasks. This console command can be simply called from a cronjob without exposing it to the web. In fact your are calling this command via the php-cli and not from a browser, the standard time limit is unlimited. No controllers should be involved for executing.
This should be all organized in an extra TaskBundle.
