Need gear recommendations

I got laid off from a non-film related job and the severance was so good that I honestly have months of free time before I even begin to stress. I want to use that time to get back into my favorite hobby: filmmaking. I'm looking to spend around $7500-12,500ish (more if it's the difference between "OK" and "amazing") for a solid short film set-up. I'm specifically looking to make a horror film. I do have some gear already:

• Sony HXR-NX80 camcorder
  • Opteka OPT-SC62FE fish eye lens
  • Infrared hot shoe light for night vision shots
• Nikon D7500
  • Nikon 35mm f1.8
  • Nikon 55-300mm f4.5-5.6
  • Kit lens
  • An old Nikon 50mm f1.4, possibly from the 1970s
• An old Sony Hi8 Tape camcorder that still works
• A basic tripod
• A 10+ year old Steadicam that doesn't ever seem to want to balance
• A few hot shoe lights (basically LED rectangles that can be brightened and dimmed, one is post-it note size, the other is the size of an envelope)

Things I need help figuring out ($7.5-12.5k):

• A camera that is capable of producing cinematic-looking footage and performs exceptionally well in low light. 4k 60+ fps would be a plus.
  • Any lens or lenses that are must-haves, if applicable
• I have no good audio gear. Any and every recommendation for a competent audio setup.
  • Headphones or audio accessories that are necessary for good audio under different circumstances (dead cat mic cover, etc)
• I have no lighting at all aside from the hot shoe lights mentioned above. It's another area of opportunity here. Taking any recommendations.

Thanks for anyone who can help recommend some gear!