Hi, first congratulations with already being ahead of 99% of advertisers on Facebook by using Custom and Lookalike audiences. If Lookalike Audiences are performing the best, the first step that you could do is expand the lookalike audience to 'Reach' to include more people. Sadly, that's pretty much everything you can do with lookalike audiences.

The next step that you could do, if you haven't already, is install the remarketing pixel on your website and set up the website custom audiences - those would allow you to remarket to everyone who visited the website but haven't converted yet and show them highly targeted ads by the path, keyword or specific URL on your website (this could be achieved through third party tools, like Perfect Audience too).

You mentioned that you have tested other targeting options, but there might be some space for improvement too. For example, have you done any split-testing by ad placement: desktop, mobile and sidebar? Have you used 'excluded audiences' so that you are not showing ads to your current clients? Have you tested numerous visual variations under the same campaign? What about looking at the reports and identifying the best performing age group and their location? Also, which bidding method have you used: oCPM, CPM or CPC?

Even though you have already tried targeting by interests, you could use Facebook graph search to identify more common interests that your clients have by entering "Pages liked by people from United States who like ABC and XYZ" into the search, just substituting ABC or XYZ with the relevant brand name or interest.

I hope that gives you some ideas. If you had any more specific questions, I'll be glad to discuss it further with you over a chat.

Cheers!
Adomas