I don't get what anyone is complaining about. I quite dug Brauer staying persistent with her push to marry Sansui. You usually don't see that stuff with "devoted servant" archetypes, and it's on the rarer half as far as isekai love interests are concerned.
And on the subject of isekai, I don't see Sansui being a blank slate or "beta cuck", whatever the hell that means. He's a sage. Like...this is literally a trait of sages. They're long-lived, and perhaps consequentially, they've purged their desires and ego.
Sansui has the sort of mental state that many religious monks strive for, with absolute patience, indifference, and humility.
Granted, he could've been a lot more creative with his power, skills, and position, but that's why he's the "boring Sword Saint".