"Android in general only slows down if you fill it up with useless **** apps and bloatware can be solved by a root and vanilla rom install.
Samsung does the job of filling up garbage for us... You don't need to worry about that.
I'm a pretty advanced user and yet I screwed up a Galaxy S5 by updating a with a ROM fRom another country, which messed up my phone that even Samsung could not fix it.
Also you make it sound so simple to hack the phone, but you won't convince me because I know that is not simple nor reliable. If you like to play, get a toy, not a phone.