It seems that https://build.opensuse.org/request/show/1075201 was included in MicroOS snapshot 20230329 and it does install xdg-desktop-portal-gtk. However, the change didn't remove xdg-desktop-portal-gnome from systems that already had it. Removing xdg-desktop-portal-gnome needed manual intervention with transactional-update. This fixed the delay in starting portal.