Ahh, sleep training..... The polite way of mentioning CONTROLLED CRYING (there I said it, the elephant in the room). I'm always very reluctant to mention it here as it's unpopular with many on bubhub which I completely respect
I followed the sleepeasy solution (a wonderful reassuring book) with dd1. It was basically a supportive how to guide and a plan of attack for doing controlled crying.
Look, it worked for dd1- it reduced her wake ups from 8+ to 1-2 a night (she never consistently slept through until I completely weaned her at 2). She became a dream to put to bed and napped like a champ. And she was so much happier (and so was I) as a result.
So I will have no hesitation "sleep training" again when dd2 is around 6 months- she is very tiny so I'm reluctant to (gently) wean her off night feeds until I'm confident it'll work. I found this guide gentle as you wean them similar to dr jay Gordon's methods.
But as I said, it's not a popular option here- I used to be against sleep training until dd1 broke me and I saw the positive results first hand.
Eta: dd1 still had bad nights/phases of not sleeping when teething, hitting a milestone or sick. Completely normal. But it just meant she had the ability to self soothe when she just wanted to sleep.