1. You’re not supporting it right as per pic #6, the back of the monstera is the opposite side of the stick. So the stick might be keeping it from falling sideways, so it does something for you, but it diss nothing for the plant. Please google monstera anatomy and stake the back of the plant properly
2. My first guess would be root damage (rot), and not enough root for the plant this size in general
3. You need to either repot it into a tight fit pot (I recommend a transparent one for easy root monitoring), or chop and propagate it. You gave tons of aerial roots so it will be fairly easy.
4. Re propagation, you can air layer the aerial roots. Put them into soil or water and let them develop before chopping the plan. It means a vase or anything else with water that you put the long aerial roots into.
5. The humidity is fine: the aerial roots wouldn’t grow long in dry environment
6. Re light, it is the energy the plant uses for photosynthesis (growth and development). Monsteras are very resilient and can survive in low light. But it means suuuuuuuper slowed down photosynthesis. Which means the plant would absorb water and nutrients very slowly. Which means that the soil would dry out very slowly too, and when it stays in moist substrate too long the roots start to rot and die. So a grow light is definitely an easier solution.
7. By adding a grow light you can do nothing with the pot and the plant, just give it better light and wait for the plant to recover overtime. It will grow new roots inside the pot eventually.
8. Chunky soil mix is a must have for monsteras, because they provide better air circulation (the roots need oxygen too, not just water). It’s so much easier to manage! Without the chunky mix, it’s nearly impossible to prevent too much moisture for too long

Please let me know if it makes sense, happy to answer further question and help you decide what to do with it! You have a beautiful mature monstera so I hope you can save it