Simple Mobile apps have been very popular among FOSS enthusiasts. I've personally been using the Gallery, Contacts and the Phone app since a few years now. It's a shame that it has come to this, will be on the lookout for their forks.
Goodwy seems to be forks of SMT apps https://github.com/Goodwy?tab=repositories. Open source, no ads, no trackers and no internet permission, unfortunately you have to either compile yourself or download them from the play store. https://play.google.com/store/apps/dev?id=8268163890866913014