You might find the LGPL used by the Wine project to be more attractive to potential contributors. The first thing I thought a few years ago when I read that your project was GPLv3 was that any code that I develop using it would have to be GPLv3. Even if the GPLv3 itself might not force that for "system libraries", the harassment from armchair legal experts would. As much as I like OSS licensing, I do not like having my choice of OSS license forced on me. I am probably not the only one who feels that way. The LGPL is a far better choice for this kind of project.

For what it is worth, that is from personal experience. I went through the trouble of re-licensing a script with consent from all contributors from GPL-2 to BSD-2 in a package in the past because of harassment from armchair legal experts. It had been mistakenly marked GPL-2 when it should have been BSD-2 to match OpenRC. It had been included with binaries under the CDDL. There was no incompatibility and the GPL FAQ even talked about the case under mere aggregation, but that did not stop the harassment from the stream of arm chair experts rushing to tell me that I had committed some egregious offense for having the package marked both GPL-2 and CDDL. Relicensing the script to the license it should have had in the first place put an end to it and created one less GPL licensed thing in the world.

Anyway, switching to the LGPL now would be easier than switching later. It only gets harder as a project ages because you start to lose contact with past contributors.

I hope more visibility can help, you can try to highlight more the need for contributions in the project homepage and in the github readme maybe