The situation you are in is common. Someone wants work without having an adequate budget. In this case $0. Sure you might work for free. But a busy security professional that could do an audit probably won't.
Making the project open source doesn't mean you will get any help for free. Sure you might. But there are countless open source projects and you are the only programmer who is currently interested in this one. Making it open source isn't going to make anyone else more interested. There isn't an unmet need for projects with non-paying work.
If the app is important to the charity organization, it's fund raisers can raise funds from the usual sources for doing it at a level coming closer to doing it right.
Good luck.
- [0]: https://owasp.org/
A good solution is to use an existing service with strong data protections to hold your actual content and then build your service to use authentication tokens with that service. There are lots of options depending on what kind of data you need to store, like Digital Ocean for a SQL instance or Firebase for JSON.