I think if you're going to do crazy hard bosses you need to lose runbacks. In general I prefer easier boss plus runback, but first sinner was pretty great as a fight to encourage needle mastery. Also not anywhere near as overwhelming and nutty as many of the fights since there is only a max of 2 projectiles that move in straight lines on screen at once (blocks of needles that move together are just one projectile with a fancy hitbox in terms of cognitive load).