Not sure what is causing this, but as long as gui-wm/sway is compiled and installed with the wallpapers use flag, the thing just refuses to start. Instead it spits out a misleading error saying "00:00:00.000 [sway/main.c:123] SUID operation is no longer supported, refusing to start. This check will be removed in a future release", yet this has nothing to do with the kernel config, elogind or seatd. I assume the ebuild is configured incorrectly...
emerge --info, any logs, a version number? I'm not sure how your last sentence follows from what precedes it.
See also https://forums.gentoo.org/viewtopic-t-1171912.html.
Created attachment 912330 [details] emerge --info (request)