That should work, except I'd use a separate instance of the app for each. If you put them all in one app like that, you'll get unexpected results: say, you open Back Door at (let's say) time 0, then open Front Door 10 minutes later. Five minutes after that--not 15 minutes--you'll get a notification that "Front door is still unlocked." Keeping them in separate apps will track each device separately, which is probably what you want.
And yeah, not all the options in Notifications let you use repeats. I'm not sure why--perhaps they just added it to the ones they thought would be most useful. With a rule, of course, you could do anything you wanted. For examples of that, I'd take a look a the patterns in this thread: