Are you funding it privately or do you have investors or other support?
What about charging a few sats a month per user as a voluntary fee (popup once per month with beautiful design and bullet points what cost you need help covering).
Discussion
Privately
I hope you find investors. Your client is the very best of them all. Also better than Damus.
You should be able to ask people for voluntarily client payment once per month and/or her a few million USD from investors.
Even a 2% fee on all zaps, up to max fee of 1000 sats per zap should be acceptable to most.
I am not sure if I want investors. I raised money before... But I do take donations though :)