Really really thanks for the comment, I am happy to interact with someone about this project :] !!
Regarding your original question, I have tried both Mutt & AERC in the past and have had an overall pleasant experience with them since my basic needs such as sending and receiving mails were being met but this was after I got more familiarised with using Linux.
Way before this when I was a newbie who was just out of high-school and was just really scared of "using the terminal", even learning Mutt seemed like a very high learning curve to me because, being honest-to-God when I say this, I am pretty slow at picking things up and it takes me many attempts to get something correctly.
Looking back, I just really wish there was a suite of terminal-tools for GUIs that are used by common people (like Gmail.com) to help me lose this fear as quickly as possible because it really is somewhat irrational to turn the terminal into some kind of boogeyman. When I say "Terminal", I don't only mean the linux-terminal as I plan on releasing Gmail-TUI for Darwin and Windows (https://i.imgur.com/4NViNeb.jpeg) as well.
With Gmail-TUI, I just want users to be able to seamlessly transition from using the Web-Gmail to using it's same functionalities on the terminal with a user-interface that they would already be familiar with: No modifications needed in any configuration files, just enter your credentials and send-receive mails using your keyboard & mouse.
Also, dev.vaayen@gmail.com is my email where maybe you can contact me in case you would like to join me for the beta-testing of Gmail-TUI. Thanks again for the comment, I had a really good time writing this reply and I am now motivated to work even harder for this!!!
Can you maybe add the option of allowing users to change the theme/colour-scheme of this TUI?