I finally bit the bullet and bothered to try to make my desktop look somewhat non-default (since I get daily comments about how bland it is). However, the wallpaper placement preferences between sddm and PlasmaShell appear different.
As an example:
…whereas kcm_sddm doesn’t provide any of those options:
I’m aware that, unfortunately (per [Approved] Plasma 6 Proposal: incubate SDDM (#91) · Issues · Plasma / Plasma Desktop · GitLab) SDDM isn’t yet a KDE project, and thus isn’t as well integrated. However, I’d just like to confirm this, since it seems like an unusually significant omission for a DE which regularly features on https://www.reddit.com/r/unixporn/search/?q=KDE&sort=top&t=all, so I expect that I’ve missed something unintuitive.