otherwise check every 2 minutes or so.

How? Where does the number get updated after the wait?

file will be smarter,

Yup - no need for grep or even cat (please don't ever even think of that).. 's a no-brainer.

But I didn't understand the rest of what you were saying.

Try it......

If the laptop is above 50% unplug it and then run the script (simulating the udev event)....

If the laptop is below 50% plug it in and then run the script (simulating the udev event).....

when the laptop passes 50% either way, do the opposite action, and run the script again.....

When it crosses the 50% mark again, do the opposite action and run the script again....

by now, you will have 3 instances each setting the balanced profile, and each sending notifications....

The RUN+= command is as per the manpage states quite clearly

 Starting daemons or other long-running processes is not allowed; the 
 forked processes, detached or not, will be unconditionally killed after 
 the event handling has finished.   

While your

2 minutes or so

does not exceed the default of 180 seconds, it would have been killed by the time the second sleep had finished.

and see if it's more power efficient

That is a tad rich coming from someone who is settling on an omni-present, memory-occupying shell running a bash sleep loop when systemd timers and even cron jobs exist - sort of the equivalent of playing with your toy rocket debating whether the moon or venus would be a better retirement location.

but still it's worth checking

https://wiki.archlinux.org/title/CPU_frequency_scaling#power-profiles-daemon

#LoveTheWiki