From which user did you ran crontab -e?
If you ran it from user dietpi, than you edited the crontab for this user. And reboot needs root privileges, so try
sudo crontab -e and edit this file and add:
0 8 * * * /sbin/shutdown -r now
(reboot command needs a time when it should be executed, so now indicates it’s executed instantly.
BTW the f irst part of the line means it’s rebooted at 8 a.m. (minute 0, hour 8 ) Change it to you desires.
did you tried to restart AGH alone to see of this is already changing thinks?
How does resource consumption looks like once you “feel” dropping performance? Did you checked htop how your system looks like?
Do you know the process using most memory? Maybe you would need to watch it over time and find out which processes are increasing on memory usage. First you would need to get clarity on what is going on with your system. Maybe there are running more processes increasing the memory usage day by day.
if AGH is increasing over the days and is growing on memory consumption, you might need to report it to AGH to report a potential issue (memory leak). But it depends how much it is growing and how your system is utilised in total.