My guess is that merely telling truth might work, as the suggestion could nudge the NPC into Bond Villain mode:
"these people are no threat, so I will reveal to them my great plan!"
"I can totally trust these people with this secret that I have been harbouring all these years!"

It does not ensure that the NPC will actually tell the truth, now. If, say, an innocent was Suggested that he reveal his plans when he actually has none, he might spin some up just to please the suggestion.

Your given wording, though, is not a good suggestion. A better one might be something like "You want to show us how dumb we are and reveal your real plans." Or "You absolutely trust me and will tell me everything you know." Your DM may actually like it, as it gives him a chance to roleplay a monologue.

Nothing is stopping you from making a suggestion telepathically

While suggestion has a verbal component to it, nothing in the description of verbal components nor the spell itself says the verbal component is the suggestion.

Therefore, as long as you are able to chant the mystic words, you can telepathically make the suggestion.

Verbal components (SRD pg. 101):

Most spells require the chanting of mystic words. The words themselves aren’t the source of the spell’s power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion. Thus, a character who is gagged or in an area of silence, such as one created by the silence spell, can’t cast a spell with a verbal component.

Suggestion spell (SRD pg. 181):

You suggest a course of activity (limited to a sentence or two) and magically influence a creature you can see within range that can hear and understand you.