I think fundamentally you can't tackle the problem the way that a traditional VC backed startup would. You've got to burn through money like crazy and there just isn't going to be the ROI multiplier at the end -- because you can't lock in your customers. Free software business models have to work on a "payment" upfront way of doing things. However, payment can be made in a lot of ways (including the "consortium" style approach that Apache software takes). If you ever want to talk about free software business models, look up my email in my profile and I'll be happy to go on for as long as you want ;-)

I think that volunteer organisations can go a long way, though -- especially if they are motivated by things other than money. Scarce skills are actually not nearly as much of a problem as you might imagine. I actually have the skills to fix your audio problems in all likelihood. In fact, I would go so far as to say that I haven't found any VOIP package that is as good as one that I built 15 years ago (unfortunately not free software... sigh...)

The key is to actually run a free software project. Signal is not free enough for me because it forces me to link to non-free code if I want to use the service. I'll be completely honest, open core sucks when it comes to attracting people who have the ability and desire to help. It is vendor lock in by design. And while Signal in not actually open core, it might as well be because of the lack of federation coupled with the necessity of using Google Play and Signal's server.

If it were me, I would rewrite rather than fork Signal because I also don't like a lot of things about their design (and I have considered it). But in the end, I'm not all that interested in writing a VOIP/messaging server. I've got another project on the go and I'm only one person. This doesn't mean that there isn't someone else with similar skills to me. It also doesn't mean that I wouldn't chip in and help with a project that I felt aligned with.

And I think this is exactly the key. You need that low barrier of entry. This is one of the absolute key things you need for building a free software project that attracts the talent you require. That is a lot more to it, but without removing all the excuses to work on something, people just won't find the time.

I think the problems you are currently focusing on are secondary. You do not need to fix them up front. They are things that you can iterate on. It's the "how do I get people excited to work on this project" that's important. You don't need to appeal to the masses for that. You need to appeal to the programmers (and potentially documenters and artists). Once you have people invested in your vision, then you can start to wonder how to expand your reach.