The main differentiator between rich people and others is the fact that rich people don't have to work. This is the biggest freedom. I figured out that I don't need to be rich to do that. I still work, but I mainly do this because I want to. I say no to things that I don't like and I've never been employed (I'm an employer myself).
I wouldn't trade this freedom for 10-20k/month (which I could achieve, but I would have to work my a* off). Deciding what to do with your time is the ultimate freedom.
edit: I worked like this in the past (making good money and having lots of stress), but nowadays I don't see the point anymore. Even if you like your work, wouldn't you prefer to work on your own visions and on your own terms?
Background of this question: Also form Europe (Austria), an EE engineer but modestly paid and overworked. I figure I should pivot to a passive income source as even switching jobs won't imporve my quality of life since companies are overly stingy on paying competitive salaries and most try ot burn you out in the long run as they try to be more competitive and government will tax the hell out of you the more you earn so I figure why bother, my healh is worth more than that.
1. Know your numbers: How much do you need to live? How much do you spend each month? How much money can you save up in your job every year?
2. Save up. Live extremely cheap and save every penny. You need enough money to live for ~9-12 months without any income, e.g. if you need 1200€ you need to save minimum €14k - no holidays and expensive stuff until you've reached your number. This is your lifeline money.
3. Save a little more: You need to invest money to make money. You need about €5-10k minimum for investments (buying domain names, themes, content, designs, ...)
4. You have saved up €20-30k? Continue the plan. Otherwise continue to live extremely frugal and increase your income.
5. There are many ways now: build websites, AirBnB, flipping stuff, writing books ... - first, you have to build something. After you've found something that works, you continue to increase the profit margin and automate the process (also utilizing manual labour). You can only do this if you already have some money, this is the reasoning behind step 1 and 2.
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In my calculations, you need to invest roughly €30-40k to make €1-3k/month in completely passive income (I achieved it with 5-10k, but I was lucky). It's definitely achievable in 3-4 years to save this amount of money if you commit to a frugal lifestyle, save up and invest it correctly (I achieved it in some months, but as I said, I was lucky).
I think it's not fair to sell you the idea that it's easy - although my journey was extremely straightforward, I know that most people struggle to even make a dollar online. But I can assure you that you will find a way if you're committed to leave your day job behind.
But please make sure that you have enough savings, I know many people who left their job and after 6 months they ran out of money. You don't want to be the defeated one who wasn't well prepared for this type of lifestyle.