I didn’t see your latest post in time (I was so eager to get Antergos working that I already gave it another go and it is currently installing, yet again) so I didn’t try the Legacy mode, although I’ll be sure to do so if this fails. However, this one looks more encouraging as it allowed me to use the “/boot/efi” already on my system with no problems! Hopefully this works!
Note: the result (which was a success) is on the next page. Thank you very much for all of the help.