Schedule reboot in the background during debian preseeding

I want to schedule a 30-minute reboot timer inside a Debian preseed so that in case of download problems etc. the installation does not hang (requiring someone on-site to reboot the machine manually). I am installing from a netboot image. The following does not work (it does not run in the background, but in the foreground):

d-i preseed/early_command string reboot -d 1800 &

d-i preseed/early_command string sh -c 'reboot -d 1800 &' ;

I think the culprit is the log-output program used here https://salsa.debian.org/installer-team/preseed/-/blob/master/preseed_command#L16 Is there an alternative way to accomplish the scheduled reboot? Bonus if I do not have to install additional software (again, network problems …)