The discussed topic here is how to raise users awareness about donations. I think that your suggestions above, specifically nagging users on their own device, are bad. Putting it like you did in the question seems strange (without a pressure on the users the OS would discontinued), because the OS would be long dead if that worked like you are implying.
The point is that you shouldn't ruin user experience to raise more money. That's crucial.
You can advertise the system and also spread awareness about needed donations on the grapheneos web (anyone who install the system would see it), you can use social media, esp. Twitter etc. I do not at all believe that as someone in this discussion (maybe you?) mentioned that 80% users doesn't know about the chance to donate or care about donations. Personally I think that if someone is interested in leaving Apple or Google and go for GOS they must care a bit, and probably tried to get some information.
I am new user but I noticed that everyday there's someone new asking the questions, starting new thread etc. it means that it doesn't look like the community is diminishing, on contraire. Some of the new users would eventually donate. I am not a mathematician but I can take a hazard guess that with stronger user base it is probable that the number of people who donate will rise.
Also I guess - speaking strictly about the money - there's a much bigger chance to persuade someone to donate by providing better user experience than to attack him with banners or pop up ads that he/ she should consider donating.