It’s about as difficult to learn UE4 C++ as Unity C# if you have no experience in either one.
Given the criteria you laid out, there is one huge advantage that UE4 has over Unity for a solo dev and that’s cost. UE4 has a lot of standard powerful tools(example: materials and shaders, sequencer, etc) that have no free equivalent in Unity. depending on what you use it can cost between $500-$1500(total, not per tool; shadeforge, playmaker, etc) to buy the unity marketplace equivalents of many of the UE4 tools that are included in the base package for UE4.
The second is source access, I used Unity professionally for 5yrs and regardless of any feature they ever add in the future, I learned my lesson and will never switch to any product that doesn’t have source access. You may rarely or never need it, but when you need it and don’t have it, you’re in for a wild ride of wasted time. UE4(and other source accessible engines) give you the freedom to know that no matter what roadblocks you face, you at least have the opportunity to address them. Black box engines do not give you this freedom, and at least in my experience, I find Unity users often changing their game design and mechanics to fit the engine, where unreal allows you to do the opposite.
That to me is absolutely invaluable, well the 2nd isn’t an argument specifically for a solo dev, I think it’s the most important argument for a game engine choice.