Can I use init in cmdline.txt to run something at boot?
BUT if you want to run something at boot, why not use one of these options:
Personally, if available, I’d use systemd. For more info:
And replace systemd? Sure, you can run anything you want. 
Because I want to do stuff with the storage devices, ie expand the file system.
Right now I am using systemd oneshot to call a script, that does a bunch of stuff.
But I have learned it is possible to call that script before systemd (if I understand it correctly) by init=/pathtoscript.sh in cmdline.txt never using a systemd unit at all.
I do this on raspberry pi os, hence the question.
edit
rc.local is an option I looked into, but the method above would only demand a few lines of code in the script I would call compared to that, since I would only want to do this ONCE, then remove everything.