So it has been running for around a week with a daily reboot and I think it's safe to say that it is working!
Maybe you need to add some pause between the lines/commands.sleep 2
I added the sleep between every command and apparently that did the trick! Plan B (as someone kindly suggested in another conversation) would have been to chain all the script in a single line with ANDs and (lol) ORs to make it fit in the single-lined input box in the OMV GUI, but luckily there was no need to get that dirty.
Still I have no idea why if I don't echo 1, 0, 1 (in that order) to pwm0/enable it doesn't work, but I am willing to live with it.
For the curious: when I echo the first 1, nothing happens. Then when I echo 0, the fan starts spinning like full power. Then when I echo 1, it slows down to what I like to think are my period/duty_cycle settings. I think 0 is supposed to turn it off and 1 to turn it on, but ♂️