I don't know if you have tried this yet, but sometimes having a lookalike custom audience that is 99% similar doesn't always preform better than the lookalike audience that is 97% similar.

If I were you I would go through AB test each of the different lookalike audiences, so try 99% vs 98% vs 97% vs 96% vs 95% and see if the less similar audiences will still give you good results, because they should also have a larger audience group for you to market towards.

Also have you AB tested interest groups? I'm not sure what your exact niche is but i've worked on campaigns where certain interest groups preformed better than the lookalike audiences.

If you're spending a decent amount of money on Facebook ads, having a professional go through and AB test your ads can be a very good idea. It's the difference between having a good ad & knowing you have a perfect ad.

When you created the Lookalike, did you slide the scale to maximum for reach instead of similarity to increase the numbers?
Who is your ideal customer and can you give me the landing page URL? I could potentially offer a couple of suggestions...

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