I'm not sure I follow what the problem is here. Why does a relationship to limit the list of Credit cards, limit the list of clients?
And do you need these join tables? (we can simplify a bit if you don't). You don't seem to need a join between client and credit cards unless the same credit card can be used by more than one client. (Two or more clients from the same company or family?) The lack of "crows feet" in the relationship linking credit cards to the join table suggest that this is not the case.
And you don't appear to need a join table between estimates and clients unless you have a list of multiple clients getting the same estimate.
Thanks Phil - I think that I've overdone it with the joins. I'll try removing them and then have another go at it. But I think I understand your points.
Note that with some business models, one or both join tables might very well be necessary. A credit card might be a corporate card used by different employees of the same company and you might need to treat each as a different client. A project might have more than one client--each might be a partner to be served by you both as a group and as individuals.