You need to be booted into EndeavourOS and add that line to the default grub and then update grub again. Then what i do if it is working right and EndeavourOS grub menu is the one controlling it i boot into each other linux os and remove os-prober from each. That way when their grub gets updated it won’t take over.
Boot Into EndeavourOS
sudo nano /etc/default/grub
add the following line if it’s not there.
GRUB_DISABLE_OS_PROBER=false
Then ctrl + o and then enter to save
Then ctrl +x to exit
Then run again
sudo grub-mkconfig -o /boot/grub/grub.cfg
If it’s working correctly boot into other Linux OS and remove os-prober from them.
Edit2: After removing os-prober from Mint and MX you can update grub when booted in each of those and it will only load the OS you are booted in so nothing else will be in the grub menu’s on MX and Mint. Then only Grub on Endeavour OS will have all the entries for Eos, Windows, MX and Mint.
Edit3: This is the way i do it on mine. Your Choice.