1) Get off the major cloud providers that charge insane egress fees. 2) Remove SMS verification. A simple solution might be the app gives you a code and then you dial in to them and punch in the code to them. Like a reverse voice based authentication. 3) Remove voice and video calling for non donating users. 3) Remove media texting until both users allow a p2p connection. 4) Remove no contact list message hosting for non donating users.
Lot of unpleasant trade offs there. But I would rank having a text based private messaging app as the top feature. Everything else is a "very" nice to have. I applaud what they are doing and the sacrifices that have been made so far.
And how are you going to convince others to pay for Signal when there are many free alternatives, including WhatsApp, which most people already have and while not as privacy focused as Signal, does have end-to-end encryption. If Signal makes people pay for voice calls, they will simply use WhatsApp, regular phone calls, or whatever is free and popular at the moment.
The success of Signal came from being very low friction, privacy is the "nice to have" feature, at least for most users. But add friction and they will look elsewhere, Signal is not WhatsApp, it doesn't have enough of a critical mass to keep users on its network.
All that will remain will be a small core of cypherpunks and people who really have something to hide. This is bad because one strength of Signal is that it is a mainstream app, making it hard to single out "interesting" people compared to those who just use it because their geek friend told them to and they like the shade of blue.