I do not think it is that complicated. The front-end sends a request to the back-end with the current selected day. This triggers a search in Immich returning all photos taken on that specific day. This is returned to the front-end and this than does the heavy lifting like filtering them to the current map bounds, displaying them on the map at a specific location. We proxy all request from the front-end through our server because of CORS issues and I did tried to avoid having to configure Immich besides creating a token for the API.

One would need to either create a specific IntegrationService like ImmichIntegrationService and then figure out a way how the user can configure that. The easiest would be that we just then call all available ones even if I do not see the use case of having multiple Photo-Servers. But it would make the code in Reitti cleaner and would not hurt if we do not configure 20 simultaneous servers :D