Use the same dependencies and their order in both systemd units.
Add additional units of services that kodi uses.
Don't use "Requires=", kodi can run without a network connection.
Export WINDOWING=x11 in kodi.service to skip the autodetection that /usr/bin/kodi-standalone performs.
Give kodi-gbm.service the privileges of input group, this allows removing the kodi user from that group's membership since kodi.service does not need it.