To clarify the 'timing options' term, what I specifically need is support for
@reboot since my project currently implements persistence this way. I'm using an older OpenWRT version from around 2021 on my router which doesn't support it but maybe a newer version does?
If not then I suppose I have to create a procd script? And then a followup question: is there a useful and up-to-date guide for creating a procd script for OpenWRT?
You would need to compile from source and enable
Compiling from source is not really a good solution since the project is designed to be used by other people, it's not a question of how to make it work for me personally.
Then the answer is 'no'.
These convenience features for the various busybox applets are disabled by default and very unlikely to be enabled in the future, you will have to find another way (procd initscripts may cover some aspects).