However, my problem is, when I try to send an email from one of the identities (Aliases) it always shows my main address with a slash followed by the identity name.
Where does it show that way? In Aquamail when you are trying to send it? That's Aquamail tells you (gives you a choice to confirm), - that it will be sending using the identity "IdentityName" with the "AccountName".
Your "main address" in this case is just the name (a label) of your account that correspond to that address.
(By the way, the account name is totally aesthetic. You can rename it to something else, if you wish, without changing the account address.)
Try sending a test message to yourself (preferably to a different account), and see which "From:" address is there.
I don't have experience with G-Suite, but I assume that G-Suite is aware that those aliases are legitimate for your account. Otherwise, it will be rewriting your "From: alias@domain " to the "From: main-address@domain".