That's a matter I did think about a lot when started building this campaign a bit over year ago. Sorry to hear that voice-over quality has been so immersion-killer for you. And again, like about landing checklist, you are not only one to say there could be some work to do.
At beginning I did some tests with human voice "actors", quite fluent English speaking friends. Quality was sometimes ok but it was clear that speaker was non-native. Also not everyone willing to try had voice fitting for this purpose. There was also a problem that every time I fixed campaign so that I had to change some voice-overs I had to reach the same "actor" again. Of course voice-overs should be done last but I can tell there is no point when campaign needs no fixes anymore. Mostly for those reasons I decided to use computer-generated sound actors with some editing. By the time I didn't found a perfect program for the purpose and had to use quite a lot of time trying to make voices sound better - not enough according to your experience.
Afterwards I found out that there is a very large community willing to participate voice-acting as you said. There is still that campaign-fixing problem but I have now seen quite many developers using this method with very good results. I just also found a much better program for quite-humanlike voice generation with natural emotions and so on. Expensive as hell of course but maybe worth the money. But I haven't been able to use it in this campaign.