The only thing that's increased my ftp is stuff above ftp.
Last year I built up to doing 3 hours of sweetspot at 89% ftp and that didn't help, but doing 5-6 min repeats at 115-118% helped tremendously.
Could be highly dependent on each person and their previous training, however. And there's typically more than one way to do many things.
I very rarely do any type of z2 rides over an hour, so the traditional "base" of 3-5 hours at z2 is a total waste for me, and boring as hell anyway, so no way I even bother with it at this point in my life.