Legally, since you wrote SWE, then you should look up "Trolöshet mot huvudman" https://sv.wikipedia.org/wiki/Trol%C3%B6shet_mot_huvudman
If you have signed any NDA's or contracts, you should have read through them as it specifically says you are not allowed to work for any competitor or do competitive freelance work. If you ahve any side businesses they generally ahve to be approved by the Business Ethics department of the company legal team, and approved by your manager as not intruding in to your daily workload or competing with your job.
So you'd be pretty much fucked if they find out. At best you are fired. At worst you are looking at a legal suit, possibly even criminal suit if they decided to say that you stole company confidential information and provided to the other company.
Once that is done, it doesn't matter if they win or not, you'll be paying through the nose legal fees, and no one will ever hire you again in the business.
but hey if you can get away with it, there is money to be earned.
Though on a personal level, consider your mental health as well. How long are you able to keep up the charade? How will the stress affect you long term?