What do you do when you max out a sims career?

My sims are not all about careers. Most of them do have careers, but I level them up to what makes sense for them and their story. I'm a storyteller at heart, so for example my sim Jacqueline is very insecure about her body right now. She's in the athletic career and still a locker room attendant (since she just started), but she goes to the gym regularly to lose more weight and get ever more thinner so she can be society's ideal. Meanwhile, she'll level up in her career so it's a double-edged sword. She does great in her career, but her personal life worsens. Storytelling like this makes play much more fun for me. It also keeps me from marrying everyone off for the sake of it.

Freelance

It depends on my story. My crime empire Sims that live in a Gothic penthouse will always be criminals. Some of my Sims retire and do the gardening career. Some raise a family or switch careers so they can complete the Renaissance aspiration. Right now I am playing a single mom in the city. The mom who is an adult now and has reached the top of the business career as an investor quit her career and went back to college with her daughter and now son in law. At first it was just the two of them but the daughter fell in love and got married. They still live in a penthouse in the city but I am building the Mom a house in the country.

Move to a new one. Mostly I do careers for the unlocks. Once done, they hold no value to me. I prefer to generate income through crafting, gardening and interest anyway.

I don't exactly play careers. Sometimes i have a sim that wants to reach the top of his career.

After (and before) that, my sim will always have a story that is 'written' on the go. I don't play by completing goals. I play by inventing stories and embracing unexpected events.

I move my focus to other sims which is usually the kids since i have my sims work and start a family simultaneously. This is not only with careers but with goals in general. Once a sim achieves the goal i set for them, i move to another sim instead of giving them a new goal. There are some cases where i felt like i wanted to keep playing a sim even after that so i just played around with them and did whatever until i felt like moving on.

I often switch careers when maxed out after saving up Simoleons to a reasonable amount. But i mostly try to match them up in a way that makes sense and it fits the story/made up character of the Sim. Like Cook moves on to Critic Career (Food Critic) or Business CEO becomes a politician. Starting a Family and maybe raising Kids normally happens parallel to first career, to have a bit of a struggle.

Depends on the sim's story. Sometimes I'll have them move to another career, but most often I'll just let them coast there until I'm ready to move on to the next generation's story.

I like to have my Sims work through a career and try to ascend to the top while creating a family legacy. At least... unless I'm playing a challenge. If it's Family Man, I do whatever fits his aspiration. (Sometimes that's just motherloding all the time) If it's 100 Baby, usually freelance painting or writing.
It all depends on what story I'm telling at that time. I like to try to climb the career ladder just because I like that sense of achievement, but sometimes that just doesn't fit in with my Sims' lives.

My sims tend to be fairly old by the time they are at the top of their careers (if they even get there) so I usually let them keep working for a bit and then they can retire and spend rest of their time playing with grandkids.