Its incredibly endearing that we tend to lean into Sanguinius' vampirism. No shortage of sensual blood drinking, tortured repression nor dramatic struggle against the Thirst in fanwork. Even though all things considered he doesnt seem to endulge in it as openly nor struggle as intensly as his sons do.
He has moments, but they are either controlled (channeling the bloodlust via battlelust and viscious war tactics) or sparse (there's obviously the bloodwine he partakes in, a few mentions of salivation at the smell/sight of blood and I count one instance of him falling for the thirst when fighting angron, maybe more but Im not well read).
It is there, certainly so, but its tastefuly subtle, there's a great deal of implication and subtext involved and, to me at least, it seems he suffers from the guilt of afflicting his sons with it more than anything.
Which makes sense of course, he is a primarch and these are passions innate to him, either by design or mutation, so only he can handle it optimaly, even if its obvious how much he is overcompensating. So having them inherited by would-be-astartes obviously yields magnified results.
I also imagine that, when handling a character whose whole existence is essencialy defined by his martyrdom, his deeds, and the sanctification of his life into watered down catholic-shame coded imperial propaganda, its likely important to honor that through writing as well, hence why we know so little about his upbringing, and why we dont get moments that might make him too relatable (aw chucks! I had a lapse in control and mauled a person! Now there's viscera everywhere, life is hard☹️).
I'd gnaw my leg off to get a book dedicated to his life growing up on baal, realy get to know him, but i feel the fact that we dont is the point.
Even his own primarch book didnt see you in his guilded shoes, it had you following a remembrancer, scrambling to unearth the secrets he hid until he was ultimately killed for it, because the sheer necessity of upholding the legend far exceeds any honor or importance that may come from perserving or exposing the truth. The legend had long outgrown Sanguinius anyway.
His story was never about him, just what he represented in it, and it leaves you wanting. You learn just enough to yearn, to speculate about his private side, the less mythical side, the vulnerable side, but never enough, you can never quite reach him, the fantasy of pursuing him so alluring but so fundementaly unachievable, because he'll always be too far away.
