Hey, my advice may not be the best since I have only started Year 12 but I am currently doing A-Level Chemistry, Biology and French (which is also a STEM and humanities mix). So far, I would say French is my most time consuming subject, I think it's totally different to GCSE as you are having to actually immerse yourself into the culture more, but I am still enjoying it! Obviously, I'm only just a month into year 12 now, so I haven't felt the actual intensity of all three subjects yet, but I say as long as you are passionate in your subject choices and you keep up with you work from day one, you will be fine. Make sure to ask questions in open evenings so you can really see what the subject is about because people don't lie when they say it's a step up from GCSE, and also see if the topics in history and the topics in French actually interest you as I have noticed you truly need to be dedicated to the subject if you want to do well. So, just do your research on each subject and whichever one you feel more passionate about, pick that.