So far I have seen all types of domains, from 3 letters to even 12, some being more usable and relatable and other domains just being random key presses with numbers at the end.

What I can not figure out to understand is myself having verified premium domains that are more brandable, usable, literally made-up words that can be attached to any business concept, looking more corporate, valuable and some even having the same exact @handle secured on every popular social media platform and still not be sold, after promoting them over and over.

How can I actually sell without underselling before I turn 80? Because by the looks of things thats how long it can take...

I think this is one of the hardest parts of domain investing. A domain can look objectively strong, but that doesn't necessarily mean there's an active buyer for it right now. Sometimes the challenge isn't the quality of the name, but finding the right end user at the right time.