The AO that you remember, the one with a large population, no longer exists. AO’s mechanics revolve around teaming/grouping to make meaningful progression. Over the years the population has dwindled and over the years patches have been introduced to address that. There are daily missions that help you get those levels due to a lack of teams or time to play the game. The legacy server isn’t necessarily legacy/RK only content. Over time they have released the various expansions for the server so you aren’t getting vanilla AO. Not to mention that server is a real ghost town. Depending on the time of day you play, you may find people but for the most part, AO is a dead game. In your situation and with the state of AO I would highly advise you not spend money on this game.
You said yourself you have a difficulty learning this game. If you play froob only (free to play character) then you will experience only RK content, no expansions. But you are still going to be in a lowly populated game and even more so with a froob demographic. But no one is stopping you from playing that aspect of the game.
In short, AO is not what it use to be if you are hoping to relive your memories of it.