Does increasing dhikr beyond what the Companions did constitute a deviation from the Sunnah, and is it obligatory to limit oneself to what the Prophet and his Companions did concerning absolute acts of worship?

The unrestricted adhkar (remembrances), whose number, time, or state have not been specified by the Sharia, are recommended to be recited frequently without being restricted to a specific number or being singled out for a particular virtue. This is because the underlying principle is the command to frequently engage in dhikr and to maintain it.

As for the restricted adhkar, they have specific numbers, times, and states that are determined by the Sharia.

What was prohibited in the narration from Ibn Mas'ud was congregational dhikr and specifying certain numbers for adhkar that have no such determination in the Sharia.