what skills one should work on and have absolutely nailed prior to considering this type of diving?
That depends on the environment and the hazards it poses, and the kind of surface support you have available.
A good starting point would be the skills you need to dive with a buddy who has the same skills you do (rather than an instructor or DM). Beyond that, you would want to have the skills to use whatever redundant air you're bringing, and the skills to deal with problems buddies usually help with, such as donning and doffing your kit underwater, or dealing with a loose camband, or opening a cylinder valve.
There is also a mindset piece, and an underwater problem solving piece.
What gear choices should be considered?
I carry the same stuff on a solo dive as I do on a buddy or group dive, except for redundant air:
- linecutter unless prohibited
- two lights
- 3' smb
- 7' smb (except in dives in small bodies of water where I am confident I can swim to shore)
- mirror
- whistle
- compass
Whether the dive is solo or not, I always have a strategy for dealing with a 1st stage failure. This could be redundant air, or a buddy's reg, or it could be the surface (on a shallower dive).
Things like [...] someone on the surface to raise alarm if you don’t come back are obvious
What good is that going to do? The chances of someone from the surface rescuing you when you are overdue are basically zero unless you are planning your dive around the idea of rescue and have someone qualified with gear and spares topside who is monitoring your dive.
but what are some of the things one might not be aware of?
I think a lot about temperature and exposure and ability to survive on the surface of the water or on shore, especially if I don't have good surface support. What if you lose a fin? If you dive dry, what if your suit floods? No practical way to take spares in either case