Not sure if this is still valid, but I solved a similar task using a “helper” helper entity. Meaning, I created one input with the actual alarm time and an additional datetime entity for each automation related to it. To keep this in sync I use a script calculating the actual times for each of the related entities whenever the entity “alarm clock” is updated. Now I can use these “helper” datetime entities to trigger all my automations with a given offset to my alarm clock automatically.