hey, i've just come across this thread and i was in a similar position to you - picking art and dt, as well as triple science
(i want to study natsci or physics, so am taking chem/phys at a level alongside dt)
i ended up getting a 9 in both art and dt, but it took so so much time and effort - especially in art. i spent like 3h a day everyday afterschool trying to catch up or just finish and improve my coursework, in like 2/3 week blocks, and then focussed on dt whenever i had spare time... i really love dt so spending an hour or two here or there typing things up for my nea didnt feel like that big of a deal. i'd say stick with it though, for your art coursework - dont listen to those vids showing mountains of coursework needed for a 7-9. in my main coursework project, i filled roughly a third of an a3 sketchbook - with my esa taking like a quarter. quality and analysis of theory is muuuuch preferred than producing an insane amount of art (but the few pieces you do produce should take like 5-10h each :'))
i definitely felt the burnout though, especially taking two coursework subjects that both have big deadlines lined up for roughly the same time (april/may of y11.) i hated working on my art, but i loved testing the limits of how far i could go with the detail and creativity of my work and seeing my end results always made me so happy. i never really cared for dt, until we started the nea which i absolutely loved - workshop is so fun, and i'd spend my lunchtimes just working away at my project because i found it more fun than chatting away with my friends (incredibly nerdy, but don't regret.) as for preparing for the exam, our class didnt't do ANY! my teacher said we'd all be fine doing like 2h on the weekend of our own reading up on theory, and memorising the main stuff - like specialised processes and materials. i'm not sure how your school will do it, but i really think you don't actually need to do much theory prep before the exam, the nea will prepare you a lot for manufacturing processes - and if your teacher is good they'll quiz you on what you're actually doing from time to time (mine would pop quiz me on what sort of saw i was using, or to recite the process i did to vacform a certain part of my product.) because design is such a hands-on learning experience, written theory isn't loads of work, and i ended up getting 90/100 on my exam! a lot of it is related to the resources topic of chemistry, which i found interesting, so you shouldn't worry about your dt grade if you put in a suitable amount of time just working on your project, and if necessary filling in the gaps of your theory knowledge, obviously, the more revision the better (it just isn't as important as the sciences or english for example!)
i don't think the lack of relationship between arty subjects and the sciences is a big deal. art prepares your time management skills, and tests your patience on working for one thing for an extremely long time (which will be useful for labwork and research if you want to go into stem) although you might not find it interesting, it also forces you to produce something of your own from a blank canvas, with no actual structure or support as to what you're supposed to produce - this gives a good balance and break between the rigour and stucture of academic subjects! also, design is the real life application of chemistry and physics, and if you are a generally creative person, being able to see the science you enjoy manifest in a interactive and physical way should be fascinating - at least in my experience
also - i took geography gcse, it was definitely easier than most, i got an 8 with minimal revision. but it was so boring, like so so so dull. i fell asleep in every lesson. it's my least favourite subject. deffo not worth! (some modules on weather and climate were interesting, but not enough of it was enjoyable for me to justify taking it) you're not missing out.
sorry if i've rambled a lot, if you want any direct advice or help with a specific issue - i'm happy to try and help!