Emily06 Hey Emily,
I absolutely think you should at least try out extension history. I picked it at the beginning of year 12 just to test it out, and ended up loving. Though it is a lot of work at certain points it is definitely worth it. It improved my essay writing skills so much, and I believe it is the easiest extension to receive en E4 (a band 6).
I know you have other major works so that might cause stress, but extension history is only a 2,500 word essay which may sound like a lot, but I sat down and wrote the entire thing in 2 days. I think as long as you do it ahead of time and not at the last minute, it won't be too stressful as it is far shorter than the majors for S&C, art or textiles and is due before trials which is a the main reason most majors are so stressful, because they are due JUST after trials.
I had a few friends in extension history who also had 2-3 major works and they said extension history was by far the easiest and least time consuming.
I hope this helped, let me know if you have any more questions because I feel really passionately about extension history 🙂