Hello Manuel,

1. There is a way to start a timer via a button. You do this by setting the "initial" attribute of the "Time" tag to false. This will prevent the timer from starting automatically when the element starts. You then define a trigger that is executed on a button press, that executes an action that starts the timer.
2. There is a way to change the default interval of a timer. However, I wonder if you mean the interval between the execution of Groups in a Timer, or how frequently you want the Groups to be polled in a Timer. In any case, you can find where you can change these values in the screenshot below.
3. To update a parameter when a Timer stops involves some extra steps. You would need the Timer that you manually started, to include a Group at the end that will poll a Trigger, that executes an Action that stops the Timer. That same Trigger that stopped the Timer can have another Action that can execute logic to update a parameter.