Solid wire works as long as it is the correct AWG, however it is MUCH harder to work with, even if you know what you are doing. I've used it, it added nothing and was harder to terminate and use in general. For in-wall, I would recommend an in-wall UL/CL rated wire.
Bluejeans isn't overkill and cost vs. what you get it is actually a good deal. That said, for in-wall runs, probably can find something similar for less than their 12/2 that does the same thing. Their 12/2 is very thin and easy to work with though.
18-20 AWG for long runs won't work for most cases (it will technically work, but isn't forward looking if you change speakers later). 14 is probably the minimum I'd run.
If this isn't in-wall, then just get some good 12/2 and be done with it. It will work for any speakers you will ever own. I just ran two 30' runs of Monoprice 14/2 for my rear Atmos speakers and it works great.