Also, Twitch, TikTok and other apps use in-app tipping liberally these days. So this is definitely a viable business model for them.

Apple says in their Developer Guidelines: "Apps may use in-app purchase currencies to enable customers to “tip” the developer or digital content providers in the app." - so I think that means you can do this and that it is a viable business model.

I am not making a suggestion of how to do anything. I am asking a question on how one is expected to do this while following Apple Developer Guidelines.