I think my issue is there are so many ways to do a thing, and I just can’t figure out a simple thing.
I have a Multiple Entities Card, with several Lights, you can either toggle them on, or slide the dimmer. Now I just want a Toggle, that will turn on the dimmer to 25%.
The issue is that it isn’t a “simple thing”, you just likely haven’t considered the questions and edge cases that “toggle to turn light to 25%” creates. Toggles have a state that needs to be considered. What should the toggle’s state be if the lights are on, but at a brightness other than 25%? Then apply the same type of consideration to the “off” state as well as the events of turning off and turning on.
The easiest way to avoid all of this is to use a button card to effect an action.
Or, if you want to limit it to something that is usable directly in an Entities card as suggested by your example you can use either a Scene or an automation triggered by an Input Button helper. Scenes and Input Buttons are both effectively stateless, so (like the button card) you do not need to worry about defining state-related behavior.