I don't think there is any convenient way to have actions be due relative to when the previous action was completed. An inconvenient way is to insert an action which is flagged or otherwise likely to be immediately seen which reminds you to put the appropriate dates on the following action. The problem (aside from the clutter) is that if you don't act on that action to put in the dates, the whole thing unravels.
Another suggestion: generally, you don't want a repeating item inside a repeating project. Either the project or the contents can repeat, but usually not both, as there are no repeat options which terminate.
I've requested the ability to have relative start/due dates on a number of occasions. If it is important to you, you should also make a request, either using Help->Send Feedback in the app, or via email to
omnifocus@omnigroup.com. More requests for a feature make it more likely to happen, though I fear there are few who have made this particular request. I have many dozens of projects where I would like to use such a construct...