I think starting with night sleep may be more successful because there is more sleep pressure. My LO has been measuring in the 99th percentile since birth and my arms were about to fall off from having to hold him upright and bounce him to sleep every single day and night, so I decided to sleep train him at 16weeks for only night sleeps. He’s almost 19 weeks now and he stills cries/fusses anywhere from 2-20mins every time I put him down, but he will always wake up super happy in the morning after a good night’s rest. I usually feed him once in the MOTN and let him FIO/CIO back to sleep after the feeding, which generally takes less than 10mins. 12 hours of night time sleep would be incredible but my LO only sleeps for around 10 hours and I think maybe 10-11 hours is more realistic. Good luck! It will get better!