with the most honorable of intentions, with the best of intentions, with no ulterior motive, in good faith, disinterestedly, above board

so much so that

(figuratively) to be a chatterbox, to talk a lot, to be very talkative; to have the gift of the gab / (figuratively) to have a sharp tongue, to be sharp-tongued

(informal) but of course! that's obvious, I can see that now! how did I not get that before?

the better is the enemy of the good; leave well enough alone

not to know how right one is (said when someone who has uttered a statement doesn't know that this statement is even truer than they thought, for example because there is another dimension to it that they weren't aware of)

what comes too late doesn't count any more

he who laughs last laughs best

ill-gotten gains never prosper

(archaic or humorous) Nothing comes out of nothing.