Hey guys,
Just an update here: it’s working now!
I really don’t understand what happened, but today I got to add Windows 10 to the bootloader menu just running sudo grub-mkconfig -o /boot/grub/grub.cfg.
Previously, I was searching about how to add manually an entry to GRUB, and saw this thread about editing /etc/grub.d/40_custom.
Well, this returned errors when updating GRUB again through grub-mkconfig. From now on I removed those entries and just ran again sudo grub-mkconfig -o /boot/grub/grub.cfg (with os_prober enabled). It detected Windows 10 and everything ran smoothly.
I really don’t understand what happened, but I am very happy everything is working now.
And, I would like to take this opportunity to thank you all IMMENSELY! You guys from EndeavourOS community are really fantastic! I can’t find enough words to express my gratitude to you all.
You always provide an amazing help, and I am really extremely happy and grateful to be part of such amazing community.
I sincerely hope to be able to repay so much kindness and help soon, by also providing help to anyone else who needs help, like me. 
Thanks, many thanks! 