Feel like my baby is truly un-sleeptrainable

Baby is 16 months today. Ferber at 6 months, always still cried hard for about 15 minutes before falling asleep. Adjusted the schedule over and over. Switched to full CIO when the check ins would make her more mad. We get maybe 1-2 weeks at a time where she goes to sleep like a dream, no crying. But always goes back to crying crying crying at bedtime. We recently moved her bedtime up a bit from 8-8:30 to now 7:30-8. Re-sleep trained once again with CIO and the past 2 weeks have been great. But randomly, once again, as of a couple days ago, she is now crying for 30-45 minutes before falling asleep. No changes to her schedule, nothing new in the routine, everything the exact same. Whyyy does this not stick with her!

Currently wakes around 6:30, naps 12-2, bedtime 7:30-8.

Is there some random trick that worked for you? Maybe she doesn’t like her sleep sack? Maybe we should get a floor bed? Maybe we should stand on our heads and do a dance?? Idk. I’m at a loss and we’re at our wits end hearing her cry every single night.

One more thing, she still nurses 2x a day - first thing in the morning and around 4pm. Does NOT nurse to sleep or nurse anywhere near nap time or bed time.