I understand your concerns, and the delay is as frustrating for me as it is for you.
I’ve thought about open sourcing the project, and I have in fact worked for many years as one of the lead developers and maintainers of an open source project (refbase). On paper, open sourcing a project to gain more traction always sounds like a good idea. But in reality, this is quite tricky to pull off successfully. In fact, it would likely mean more work (not less), at least initially. Also, gaining agreement between a diverse group of developers with different goals in mind can be quite demanding, even reviewing ready-made pull requests and trying to integrate them (w/o loosing track on the app’s original goals, and making sure that no bugs are introduced elsewhere in the app) can be quite tricky. I’ve been there.
But usually it’s even much worse: Smaller/niche open source projects hardly gain any traction at all. It’s hard to find any co-devs at all, even more so some who share your vision and who are willing to commit to the project long-term. With refbase (which was quite popular at its time), I was lucky and found at least one dedicated co-dev. There were also options to donate, but we did never receive anything. I’m not saying, it’s not possible. But based on my experience, it mostly works for very successful projects which are of broad interest.
That all being said, I still very much like the idea of open sourcing one’s code, if only to facilitate public archiving and to avoid lock-in as much as possible.
I’ve also been thinking about starting a companion iOS (or even web) app as an open source project from the get go. However, someone else would need to take over the roles of lead development and maintenance.