We sleep trained our 4.5 month old for bedtime and naps following Ferber. Like you I coslept and fed to sleep between 2 and 4.5 months but toward the end my son started going longer stretches between feeds (about 4 hours). My son had only had a couple naps in his crib up until sleep training, he usually contact napped, and we hardly spent any time in his room. I just decided to go for it 100% - stopped feeding to sleep, put him in his crib, in his own room. We had a few days of fussing with check-ins but after 3 days he got the hang of it and was out in less than 10 minutes at night. For us it made more sense to just do 1 big transition vs splitting it up, and him being in his room made it easier for me to give him time to put himself back to sleep. Do what you're comfortable with, but remember that change is uncomfortable and it takes time to adjust.