The latest version of the Immich photo app now supports casting to Chromecast. I just tested and it works on /e/OS 3.01.
This is what PixelUnion writes:
Google Cast Support on Mobile : Immich now supports casting photos and videos directly from the mobile app to Google Cast-enabled devices . This builds upon the existing web casting feature and does not require third-party resources from Google.
Maybe this is something that could be made to work on other apps? Or is this app-specific?