I'm aware of the LineageOS for microG project, however apart from the aforementioned preinstalled set of apps, CalyxOS includes many system patches shared by other projects such as GrapheneOS, which include privacy enhancements on default AOSP DNS fallbacks, connectivity checks, NTP, and more.
I would also like to mention that Signal does not require microG or Google Play Services for auto backup (or notifications). The backup files are stored locally on the device and can be easily synced with Syncthing, Nextcloud, etc.