I'm working in Switzerland as employee. I'd like to start a second job as freelancer, would be an online business. It's not in the plan to earn big money. I believe I would earn less then half of what I earn with my employee job. It's still worth it because the required work effort is reasonable. But I checked and it seems to me see that SA or Sagl have an excessive cost for this kind of earning.

What's the best formal way, here in Switzerland, to set up individual business and pay taxes, for this kind of freelancing?

If SA and SarL are too expensive, too bureaucratic, there is always Sole Proprietorship. Establishing a Sole Proprietorship is as easy as writing your first bill.

Tax is pretty simple: Your net profit is added to your other income and taxed as such. If you are on tax at source you would have to file a regular tax return.
https://www.kmu.admin.ch/kmu/en/home...ax-exempt.html

Inform your self about the really minimal bookkeeping formalities.

In addition you will have to register with your canton's social security office and pay AHV etc. https://www.ahv-iv.ch/en/Contacts/Ca...sation-offices

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What?

You also need to check your employment contract that it would allow you to be freelance, especially if it's going to be in the same field of work.

Additionally there is/was a similar process at two places I worked - one was obligated to report secondary employment (Nebenbeschäftigung). It wasn't prohibited, but had to be reported. An article I found quickly online about secondary employment (in German).