Hi everyone, sorry for later answer.
Thanks everyone for their answers!
Thankfully, I think it was (once-again) the dirty carb, atleast for now - it works (as much as the bad carb design allows it to).
I opened it up again, cleaned it throughout, completely took apart the white "cartridge", gave it a nice clean. After that, it runs okay. Only time it stutters is when you just start it up, but for like 3-4 seconds, once or twice, but then the RPM even out. It again starts to drop and rise RPM when it gets hot, but it isn't even close as it used to be. It runs fine even with the breather tube on.
I really feared it was a vacuum leak on the gaskets, as some of you suggested. Thankfully, they're okay (I really expected them to be bad since the process of opening the carb itself may damage the bowl one). Since no parts available in my country for this specific carb, I'm glad they're fine.
Also, someone filled double the oil than it should've been. I brought it down to a normal level.
I stil don't know how it runs under load, as grass stopped growing.
Here's a video preview of how it sounds now, not perfect (turn sound on)... but atleast it runs! Thanks everyone for your help, I really appreciate it.