Honestly I think it really depends what you're good at. I don't take STEM subjects personally, I take essay subjects/ arts, but I have several friends taking the subjects you named.
As far as workload goes, honestly, I am more often drowned in work than my science friends. However, their work is more concept based than memory based. Half the time once they understand something they don't have to spend hours hammering it in like I have to do in order to memorise every tiny detail, they just do quick refreshes before tests on method and process etc.
Any a level combo is going to destroy your life if you want to do well and have straight As, but if you are good at the subjects you're taking then my advice to you is not to worry, you'll be fine.
One of my closest friends takes maths, further, chemistry and CompSci, and he is doing really well across the board despite doing basically nothing in most of his frees. If you play to your strengths then you will most likely succeed.
Good luck!