Could you maybe offer me some tips on how to go about it in a smart
If your self-assessment as to your value is correct, you hold all the leverage. You are extremely fortunate, because very few people are truly in this position, even though many believe they are.
You can go to your boss and demand a raise of your choosing. You can threaten to walk immediately if you don't get what you want.
Or, you could take a softer approach, and just discuss a raise with your boss. You could point out how valuable you are. You could talk about much better you are than freelancers. You could point out how you "literally** hold the company up". And you could indicate what you feel would be fair compensation for someone with your value to the company. Then you could talk about what you might have to do, in order to get that raise.
If your viewpoint is shared by your company, you will quickly get what you deserve.
On the other hand, if your company doesn't share your viewpoint it could be risky. At best, they could see that they need to ensure you aren't a single point of failure, and start finding ways to spread your work around. At worst, they could take you up on your challenge by getting rid of you and replacing your work with freelancers (or new hires).
It has been my experience that nobody is irreplaceable. But only you are in a position to assess your true value. And you have made it clear how you feel. So you should probably just go for it.
** Note: I'm not sure you understand what the word 'literally' actually means.