International Baccalaureate Watch
As far as time management, no matter what way you look at it it's a pretty intensive course. I had about 5-6 free periods a fortnight in IB as I recall, and everything else was lessons. The teachers didn't really speak across subjects, so you could well end up having a ton of work due in one week if multiple subjects had assignments due in (although usually they would get a sense of other subjects having stuff going on when they set work and the whole room collectively cringed, and sometimes shift the deadline if it wasn't an externally assessed piece of work). You'll be doing some kind of homework most nights, especially if you are doing art or a similar creative subject. If you are sensible you will use the summer to do as much externally assessed work due in at the start of the second year as possible (e.g. extended essay, world lit essays if they are still in the English course, etc). I did not, and so ended up sleeping very little in the first two weeks of year 13.
If your aim is to pursue a STEM subject at uni, especially subjects where A-level FM is beneficial, I'd probably realistically suggest taking A-levels so you can take A-level Maths and FM (which covers more material than HL Maths these days), as well as the science(s) needed for the subject in question. For "arts" subjects (i.e. non-STEM subjects) I think the breadth of IB is however beneficial; similarly for medicine/dentistry/vet med, coping with the high volume of work will probably be good practice for studying those degrees, and from what I've heard the breadth covered can be useful (someone I knew from my school, in a higher year, went to Cambridge to do medicine, and commented that she ended up teaching all her friends how to write essays for their course, because she of course had done it for two years in IB in her group 1/3 subjects whereas all her A-level peers on the course had mostly just done 3 or 4 STEM subjects at A-level).