I am thinking of switching form english mainstream to english language 3/4 but i dont know if this is advisable. By doing this, will it overload me with homework and studying?
There have been a number of people who haven't studied Eng Lang U1/2 and jump straight into Eng Lang U3/4, who end up doing just fine. They do usually take some time to adjust to learning more metalanguage and some of the concepts that are transferred over from U1/2 (e.g. subsystems), but, overall, are not very hindered at all, given that most schools usually start from scratch when teaching Eng Lang U1/2. I'd encourage you to check the study design if you haven't already - both to know what you'd be in for and to get yourself acquainted to the different subsystems, metalanguage and jargon used in English Language U3/4. The other thing you'd probably have to do is make sure you're even able to join an Eng Lang class due to timetabling (assuming that timetable blocks are already set at your school).
By doing this, will it overload me with homework and studying?
Depends on the person. Usually the main catch up at the beginning are things you missed in U1/2, which I mentioned a bit about above. Most of that can be done within the summer holidays, if you play your cards right.
I am a stem student so i do feel like i will be able to pick up on things easier and quicker (hopefully haha).
As a fellow STEM-favouring person and ex-Eng Lang student, I think that generally STEM-favouring students = better at/ will enjoy Eng Lang is a bit of a misconception. The correct way to approach it is more "Do you enjoy linguistics?" and "Does the study design match your interests and preconceptions of Eng Lang?", which is something you'd be able to tell with a read of the study design for Eng Lang 2020.