I don't read a ton from the Haikyuu!! fandom these days, but back when I was deep in it, I noticed Daichi and Suga fics often revolved around a core of quiet, steady devotion. It's rarely explosive drama with them. You get a lot of 'established relationship' slices of life where the central conflict is just… managing adulthood together. Suga stresses over lesson plans, Daichi comes home late from work, they make tea and talk it out. The emotional beat is in the reliability, the knowing looks across a room full of friends, the silent agreement that they're each other's anchor.
Sometimes authors explore the flip side of that—the fear of being boring or taken for granted. I've read a few where Suga wonders if their relationship lacks passion compared to, say, Kageyama and Hinata's intensity, and Daichi has to reassure him that their kind of love is just as valid. It's a more mature anxiety, less about 'do they like me?' and more about 'is this enough for the long haul?'
Honestly, the most common thread I've seen is healing. Both of them as the responsible third-years, carrying the team's worries, so the fics often show them finally allowing themselves to be vulnerable and patch each other up. It's soft, domestic hurt/comfort, and it hits a specific sweet spot.