I don't think you overthink it, I think about it too! Like I have mentioned before, my characters don't always explicitly give consent in the story.
I think for some of my characters there is an implied consent where there is an arrangement, and they have the ability to stop it, even if I don't write that into the story.
Those would be the stories that are just about normal people in a normal world, and I would never want anyone to think that the top just decided on their own that they should be allowed to spank someone.
Though I also have a 1950's story. Mine was in the anthology, Twelve Seconds, where the spanking is not really consensual. That was more "my house, my rules". But I would never support that mentality in real life.
The stories I have written that take place in a different world that seems more old fashioned and historical don't really have consent.

So yeah, there you have it. Fiction is fiction, not real life.

PS.
The one rule I have is that I would NEVER write about a child getting spanked, and nor do I hint at any corporal punishment before they became adults. It is just something that I feel very strongly about, and I don't even care if it is fiction. I will not do that.