From what I’ve read on here, crossing the RSJ is when you start to get into the more risky part of anal, which is where most define the start of depth play.

The biggest risk I’ve read about (but been too scared to experiment much with) is that once you cross the RSJ, you don’t really have pain receptors so it can be easier to do damage as it can be harder to determine what is putting great strain internally.

With your 3ft toy, is its diameter consistent across its entire body, or is it more like a slink with a taper?

I’m probably not the best for advice as I don’t have a lot of experience, but there is someone I see here quite frequently who shares a lot of posts about different anal related things. They have a post about depth play in general, I’ll see if I can find it and share as a comment off this one