I think you and I are talking past each other or more specifically using slightly different definitions or understandings of terms. Hitler's rise to power occurred over a decade-long stretch of campaigning starting in the early 1920s. So yes, in terms of time he was well established, but some context and consideration must be made. First, he was considered as non-viable by basically every pundit of the time, and the ease with which he was jailed reaffirmed that view. Secondly, the actual shoot from him being released from prison to obtaining power was much shorter.
Where I think you and I are talking past each other is with the term "normal political sphere". I take it from your posts that you are reading that as "Outside of politics in its entirety", yes?
If I understand that correctly, then let me say that's not how the phrase is being used. Rather, the normal political sphere is the mainline, mainstream political beings. Your Pelosis, your McConnells, your Cruzs and your Harris'. People with long-entrenched natures in the system. This is your normal political sphere, the sphere in which people generally feel safe and secure with their candidate. Examples of being outside this are your AOCs, your Trumps, etc. People who appeared on the scene and gained a personal following. Who were known even before then, as Trump himself has spoken on political topics and even ran before.
I am not saying the person would be a total political outsider unknown to anybody, but that he'd come outside the normal channels of politics. They are an outsider to D.C. This is also why we won't know until it happens. While they won't be a totally unknown player, their status outside the normal political sphere means they haven't really be considered, they haven't been analyzed. Their message hasn't been considered.