Appeal to the school cheerleaders in every way you can. They make a big difference. We offered tryout prep clinics/classes, and even tumbling classes exclusive for middle school and junior high teams that we let the coach oversee. If you do that though, remind the coach who is in charge in your gym Bc we had boundary issues in the past.
Another cool thing is to offer "toddler" classes (if you can handle that; it can get rough honestly). We did a class before school got out for the big kids and it made the babies (and their parents, who you can invite to join in and make it a "mommy and me" class) feel very special to get to use the big floor.
Also, have "bring a friend" practices. Everyone on the team brings a friend and they get to play around in the gym and see how a practice is run. Maybe even offer an incentive for the current members to get new girls there. (Our gym actually offers a small discount off your monthly bill if someone new registers using your name.)
Lastly, and this one seems obvious, but we did a summer day camp that we advertised for almost solely through social media and made a lot of money and got many new girls (one of which ended up having to step in on one of our teams at the start of the season due to injury; you never know who will walk in your door!)
Good luck to you! I know all about the small gym struggle, but it sounds like you know what you're doing!