One sim, all careers possible?

i think james turner on YT has done it, i know he had an ultimate sim challenge where they max out all skills, all aspirations, etc.

Lol I'm lucky if I can get one sim to complete one career.

It's possible to complete all careers with Discover University (DU) and you don't need to turn aging off. Sims were limited to one branch of a career before DU but university degrees can unlock the other branch, making it possible to complete all careers. There are plenty of ways to avoid aging without turning it off completely, including reward potions, spellcaster potions, and vampire immortality.

I think the most I've done with one Sim is around 5 or 6 careers. I just don't stick with any one save long enough to get through all of them.

I think it is possible. What you have to consider though is switching career branches. Let's say you completed the writing career in the author branch and want to switch to the journalist branch. You can't just quit the job and then select the other branch. That would have been too simple and straightforward.
The only two ways I know is by getting the corresponding degree in university or by using mods. Mccc lets you reset the career branch.