Gear for solo: In my case, is the exact same as my gear for buddy dives. In the environments I dive here in Hawaii, entanglement risks are very small, I stay shallowish, and all my dives are a fairly short surface swim from shore in good to excellent conditions.
Typically no redundancy in any piece of gear, except a 2nd knife. The surface is my air redundancy.
Skills for solo: I might be the exception in this opinion, but I'd say it is a mindset, not a skillset that is different for solo. Can you plan and safely lead a dive at your current skill level? Could you plan and lead a dive at a new location?
Finally, I think clearly understanding the difference between "risks" and "outcomes" in and around the water while solo versus with a buddy is important.
We all try to plan for that exceedingly unlikely regulator failure that leads to a castrophic total loss of air, while entangled in a fishing net and fighting off a giant squid.... And we overlook the mundane but far more likely slip and falls getting into or out of the water with nobody around to help, or a muscle cramp, or illness, or goofing with navigation and missing your exit point....
You need to think through some of the common pitfalls and how you might avoid or possibly handle them without the assistance of a good buddy.
But other than that.... be safe and have fun.
Best wishes.