My weakest function is probably Ti (and fact it usually is on functions tests, and I just KNOW it is from interacting with xxTPs and my understanding of Ti) and I know it's a shadow function for me.
If Si is that far down for you, I'd guess you to be some kind of NJ, because an SP can still show some Opposing Personality Complex and Critical Parent with the first two shadow functions with Si.
I tend to think of Si as my Critical Parent function "I am better at remembering this correctly than you are." "I'M the authority on factual data, and you have dwelled too much on the past and should move on." [I feel bad about the past, and project it onto you]
I do think it's interesting, still, that ENFPs are supposed to like idealizing using Si with the Anima or Animus as a way of enjoying nostalgia together (I can tell you I have an almost pathological attachment to the 1980s, and have been involved in some way with more than one SJ who loved the 80s in one way or another as much as I did, in cultural artistic form via music or movies or even "ooh we had this candy when we were kids," for example my ESFJ ex used to actually go out and buy me Blow Pops, and the candy he liked and he got us packs of Garbage Pail Kids and had them collected in an album like baseball cards, and we'd sit and watch Vh1 "I Love the 80s" together eating our candy, then watch some 70s/80s horror movies, and just be totally so happy together doing that).
But I think I use far too much sensing to be an ENFP, it's not rejected enough in me, it doesn't make sense, it kept troubling me when I thought I was ENFP, especially since my writing has such an earthy quality and I've always been really good with sensory detail.
I've actually come up on functions tests as Fi/Si before, but other times Se will be higher, so it really depends on the test.
I also have a really good grasp of Ni, much too well for it to be a shadow function, I'm pretty sure, but not strong enough for it to be my dom or aux, and I've had Ni dom types confirm that I am indeed talking about Ni.