A great read Erika... and not easy question to answer; find myself internally debating "sincere" vs. "truthful." Are they the same, all the time? We may sincerely express our concerns to our boss, but we may not be directly truthful if it means we'll be out of a job tomorrow. Sarah is fond of using the term "situational ethics."
I tend to be very sincere although I am also a recovering people pleaser... to some extent. To a different extent, I am a recovering hermit, bypassing the whole issue by simply avoiding many interactions (quite voluntarily)... I have no real axe to grind, and no flag to wave for a cause; a tendency I have had since childhood-- preferring my own company to that of others, and thereby bypassing much of the BS of the world; seeing it, recognizing it and deliberately saying "I choose not to be part of that."
I could ramble at length here, but I shall refrain...
Sincerity was a challenge in the HSP community (just an example) where many really needed to be told that they were hiding serious types of mental and psychological illnesses behind a convenient label that allowed them to not deal with their issues by hiding behind a convenient label. I got tired of being "the bad guy" and I got tired of being "told off" because truth was disturbing... so I stepped away. Is that a lack of sincerity? Allowing lies to live on?
Beats me...
Peace and bright blessings!