Leaving this here as a note to myself, but maybe someone else gets some use out of it.
This fixes KDE Connect's presentation overlay thingy which forces itself into fullscreen by default.
in .config/i3/config, add this and adjust for your display:
for_window [class="kdeconnect.daemon"] fullscreen disable; floating enable; resize set width 1920 height 1080; move absolute position center
If you enabled transparency in picom, add this to blur-background-exclude = [] in picom.conf:
"name *= 'KDE Connect Daemon'"