I tend to think when it comes to matters of honesty or otherwise talking about behavior that works towards some shared epistemic ends (i.e. pro-social behavior), the main issue is whether or not we see evidence of deception.
My reason for focusing on deception rather than, say, truth or facts is that I don't think we can reliably assess those things to a fine enough degree to not get stuck in a debate with infinite regress. But even if you disagree with my epistemological stance, I still think getting honesty separate from questions of facts helps because most of what I think we care about in terms of honesty is issues of how we relate to reality and the facts we think we know about it, rather than the facts themselves. That is, we want a notion of honesty that allows us to make honest mistakes, so to me the way to do that is by moving away from directly looking at epistemic actions and instead looking at actions that inform epistemic behavior.
Thus I tend to think of honesty as the opposite of deception. If in deception one is trying to confuse or mislead others as to what one believes the facts are, in honesty one is trying to deconfuse and show others plainly what one believes to be true. Honesty is, in this way, a kind of virtue we can cultivate to be straightforward and upright in our presentation of our beliefs, not hiding and distorting things to purposes other than seeing reality without hinderance.
To add more subtlety, I think there is also an active/passive component to honesty and deception. Sometimes people are aware and actively trying to deceive, like the villain in a plot, and other times they are unaware and passively performing deception without intent, like when people forget things that are uncomfortable for them or hidden beliefs that a person wouldn't necessarily endorse warp their perspective such that they can't see things as they are. This is not to make a moral distinction, although I suppose you could on this basis do that, but instead to point out that deception is often sneaky and even if a person is not actively being dishonest they may still not succeed at expressing honesty because of passive deception that performs through them.
Total, radical honesty, then, is just what happens when we stop even passively deceiving ourselves. Quite the virtue to strive for, but in the context of something like epistemic trust, it helps make sense of why some people are more deserving of trust than others, even if no one is actively trying to deceive.