When to start sleep training?

Hello, my son is 15 weeks old, so he will be 4 months on the 11th. I want to start sleep training, but I’m not sure he’s old enough? I have an almost 3 year old and I think I sleep trained him with the Ferber method around 5 months. Here recently, it has been a STRUGGLE to get my baby to sleep at night. He needs so much rocking and soothing and then usually has a false start. I feel like I’ve been spoiled with my older son, just being able to lay him down for naps and bedtime and he just goes to sleep on his own.

So is it too early to start the Ferber method? I want my baby to learn to self soothe, but I don’t want to make him miserable if it’s just too early. He also takes a pacifier but he absolutely sucks at holding onto it. He’s started to get a bit better these days, but if he’s crying, there’s no holding onto it. I’m afraid that is going to make things harder if he doesn’t have that help him soothe.

His daytime schedule also isn’t the greatest. I feel like it’s so hard to keep baby on a schedule when you also have a toddler you’re chasing around 😅. I try to do 1.5 hour wake windows, but usually by the 1.25 hour mark, he’s so upset and ready for a nap. I follow eat/play/sleep as best as I can as well. His naps are pretty terrible too. Sometimes I can get him to sleep in his crib, but it will be a max of 45 minutes. Any good nap he gets is usually one in the baby carrier attached to me 🙃. I’m hoping after sleep training at night, we can work on naps.