If I play exclusively single player games, is there any real benefit for me to get this? Seems like they have a back catalog of “free” stuff if I subscribe?
Buy it when they have the free games you want. Otherwise, sounds like you don't need it. I will buy it when I get my PS5, because I'll need it for the little multiplayer I engage in. The cost is minimal, considering I pay like $15/month for HBO Max, and I've only used that to watch a few episodes of Lovecraft Country.
I don't think there's a limit to how much you can stack realistically, because that idiot on Twitter sat on his controller and bought PS+ through 2030.