Hi ThorstenV, your description is perfectly understandable.

Their is an option available in Bar Settings (Function key <F5>) to slow down the song (ritardando)

Unfortunately, it only works at Bar level, not Beat level, but may deliver what you meed.

Near the end of your song, select the required bar / measure, and press function key <F5>, and change the tempo (based on absolute or percentage value) as shown:

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