Well, it looks like everything went better than you could have planned. I am so happy for you.
I wouldn't worry at all about his new self soothing technique, his thumb is so much better than a pacifier, you don't need to come in the middle of the night to insert it in his mouth. And I think, it's better for him to be in charge, too. I honestly think that thumb sucking is a better habit than pacifier, even in older children.
I still think that he didn't want to nurse because of a new routine. My dd also refused to nurse because she figured out that she won't have boobs for going to sleep. When I went back to nursing after a bath, same story as yours, she gladly had it. It could be a coincidence that at the same time he has found his thumb, but I think you don't care what is the reason as long as your baby is happy and sleeping well.
At 6 months, you want him to still take breast first and then solids, you may offer solids 30 minutes later or one hour later, what works better for your little guy. This way you ensure that main part of his diet is your milk, this should stay this way up to 10 months or so, but many babies rely on breast for much longer, even up to a year or more. I don't remember reading it in Tracy's first book, I haven't read a new one so I don't know what she had in her mind.