During that time, you can use your action to end this effect and cast the suggestion spell on the creature without expending a spell slot. The target automatically fails its saving throw against the spell.
Some of our group interprets this as Suggestion being cast "invisibly", so any observers would not see a spell being cast. Others believe using Read Thoughts itself is not obvious to observers, but that when you do cast Suggestion, it is as obvious as if it were cast normally.
Additionally, for Counterspell purposes, could this Suggestion spell be counterspelled?
Searching for this only yields results about whether the Channel Divinity feature itself is spellcasting, which it is not.