FWIW, I have always said "F stab" (for over 25 years now). I doubt I have never even heard anyone else say the term at all, though, so I am a glorious outlier ;P.
I do the same. I also say et cetera in my head but E T C to other people, and soo dough in my head but soo do to others.
I wonder how many other people code-switch in this way to try make themselves better understood, when actually we would still understand one another if we just used our internal pronunciation?
I pronounce etc in my head as “ets” as if you’re about to say “et cetera” but stopped short. I read etc. in print the same way and have since I was a kid.
I suspect that split pronunciation between head and mouth is very common. I do the same thing with some words, like giblet, which in my head has a hard g, but when I say it I say it with a soft g.