I don't think little differences like having a routine vs not, co-sleeping vs not, BLW etc etc make any difference at all to later outcomes. Although I do think that reading things like that when my DC were little did probably help form and shape my attitudes towards them as people and towards parenting in general. I do think that the general "split" when you're talking about really early parenting stuff is about being parent-led and in control vs baby-led and more observing, waiting, curiosity and I think those values have served me well all the way through parenting. Whereas perhaps parents who expect to be in control find it more difficult as DC get older? Who knows. You could say it goes the other way as well - that being too baby-led or child-led makes teens/young people less resilient or more anxious because if a parent is steering them and is more demanding, that tends to have better outcomes. I know the research says you essentially need both warmth AND demandingness.
It's more the overarching patterns, isn't it? Attachment isn't built by things like co-sleeping and breastfeeding, it's built by responsiveness and consistency of caregiver as a general thing.
Sleep training in itself isn't going to break attachment if the caregiver is responsive and consistent the rest of the time. OTOH sleep training as a decision because you are not physically able to be present and you have to keep leaving the baby with multiple different caregivers is probably more indicative of a pattern which might lead to an insecure attachment. But I understand that as long as they have A secure attachment, it's generally OK - it doesn't have to be mum. It could be dad, grandparent, nanny etc.
I did all the AP stuff but I never thought it was the only way to build attachment. TBH looking back on it, I know AP in itself isn't really a thing people talk about now, but perhaps the more "crunchy" style is how it's called now - this is really nothing to do with optimising for attachment because attachment isn't so fragile that you need to optimise anything, it's incredibly flexible. It's more to do with optimising for the best conditions for breastfeeding, which can be worth doing esp since our society tends to work against optimal BF functioning. But again it's not as though BF is doomed if you don't follow Sears' approach. It's possible to do it without following everything there but I do think it probably is the optimal set up and that might be useful information for some people.
I don't think BF makes a massive difference to health outcomes long term in a country like the UK, where we have abundant clean water, modern baby formula and electricity. It does make a difference and I think it's worth supporting but it doesn't make so much of a difference that it overrides other benefits such as if BF is making the mother ill with stress - in that case, it's likely not worth that trade off.
A really good book about the actual science of attachment if anyone is interested is The Power of Showing Up by Dan Siegel & Tina Payne Bryson.