So I have the business as usual bistro and find that every day my chefs get replaced "cause they couldnt get to work" which gets annoying after a while so I take it that SP grants them careers, which makes them unavailable to be chefs. I therefore scouted around and found nras careers, and specifically nraas unemployed module, which aims to protect sims from SP. However, I noticed that it also mentioned that nowadays its possible cheating these sims into self-employed careers to get the same effect since they're technically "home" employed and aren't forced into any special behaviours like going to any rabbitholes or perform on any stages etc. (I take it this module was made before ambitions). Hence, I wonder if people have had any long-term success stopping chefs getting constantly rehired by placing them into self-employed careers, otherwise I might eventually try the unemployed module if that perhaps work better than just self-employing these sims.

Thanks for any help.

The differences between Unemployed/Self-Employed are really just down to the roleplaying aspect.
Unemployed is a good cover-all, has a few flavour titles like "Entrepeneur" to choose from, and is perfect for when that's literally what you had in mind for the sim- not employed.
Using a self-employed career works exactly the same in terms of preventing SP from pushing them into a new one, with the bonus that it can be a little more personalised to what that Sim actually might be doing with their time.

Which method is 'better' is up to personal preference, and there's no drawback of having both ingame.

+1 to the suggestion of Business as Unusual for managing the staff a little better.