Good on you for the thorough research, and I like your logic. Things change fast, and it's good to have backups in place. Overcast would probably be my choice if using iOS. I wish there were more open source and privacy-friendly versions available.
-M
No stress! I was on iOS for a really long time, mostly out of entrenched habits, and trepidation into jumping into something like GrapheneOS. However, I did end up making the switch to Graphene, and arguably with the most restrictive settings (no Google Play Services, no Aurora Store, etc.). There was a small learning curve, maybe about a week or so, but in hind sight, it's one of the best privacy decisions I've made to date. Feel free to reach out to me on Signal or one of emails if you have questions or want to talk more in depth about it!
-M