After baby moved into their own room we just used an audio baby monitor. We never used a time basis for crying, but a severity basis - if baby is getting too worked up and upset, they shouldn't be expected to calm themselves down from that, maybe ever. People rely on heavily other people's emotional support even as adults, that's a good thing.

I know a few parents that are struggling due to co-sleeping arrangements. Not sure if it's because the sleep training they did backfired, or if they never did anything besides co-sleeping. I can sympathize with those parents, as I could easily end up in that situation or worse. Therefore, I am not judging them (and I would appreciate if others, who think they are so smart, would stop judging me).

Young child(ren) want to know that we are either A) actively taking care of them, or B) Doing something that is needed in order to take care of them. They also want to have predictable and consistent treatment and responses to what they do, probably regardless of age.

They are perfectly fine with being left alone for extended periods of time, as long as we can convince them that it's for their own good and that they haven't been abandoned.

This is one of thousands of topics related to having children that could be discussed and read about in a similar form: "Studies have shown", "21% of parents" etc. ad nauseum.

What's missing is a discussion of the deeper issues at the heart of this and many other conversations relating to parenting:
A) Intergenerational knowledge transfer
B) Mother or father as a core part of human identity
C) Confidence and common sense

Anyone reading this looking for answers, I'll give you my 2 cents as well: What you need above all else is the confidence that you have the ability to figure out and decide what is best for you and your family. Anyone else's advice is just ideas that you can consider, but you need to believe that you will be able to decide (with your partner if you have one) what will be best, and your belief and confidence will go an immensely long way towards making it work.

We never read any books or worked using a predetermined schedule, but got our information from baby, grandparents and our own good common sense. The following worked for us well but would have gone very differently depending on any circumstance:

0-1 months sleep on mom in bed, 100% spotting during co-sleeping to avoid smothering - work on laptop in bed. This motivated us to stop co-sleeping ASAP.

1-3 months get sleepy or go to sleep on mom, transfer to bassinet next to bed, cry immediately or eventually wake up crying, pick up and feed if hungry, sing, read, etc, repeat over and over again. Mom is motivated to help baby sleep, she gets to sleep if baby gets to sleep. Baby sleeps better and better on their own, feel bad for baby alone in the bassinet but life's tough, need to get used to it eventually.

3-6 months same as above, sleeping well in bassinet, progressively longer stretches until only waking up once in the night for diaper change and food

6 months: stopped sleeping soundly in the bassinet, move into other room in crib, cried for 20 minutes then went to sleep. The first day, listening to that was for some reason harder than any other day. We wanted to go to baby but knew that it would be better for them in the long run if they could handle us not being there.
6 months + 1 day: cried for 10 minutes then went to sleep, slept way better than when in the bassinet same room, probably due to lack of snoring noise

6 months onwards: sleep in crib/bassinet 100% of time, no real crying when going down to sleep unless sick, hot, cold, overtired, or hungry. Falling asleep on mom or dad is nice but nowhere near as comfortable as in the crib due to being held vertically.

Things could have gone very differently, maybe baby wouldn't have wanted to go to sleep by themselves as readily and co-sleeping would have lasted much longer, and we would have made a whole different set of decisions and that would have been OK.

I would be highly motivated to stop co-sleeping ASAP for a variety of reasons, none of which are new. How do naps even work with co-sleeping??

Not sure how you get an older child to stop wanting to co-sleep or have someone there with them when they fall asleep after being used to having someone there. Seems like it would be difficult, but we would either accept the situation or figure it out if that were the case for us (and it may very well be in the future). I am not sure how I would deal with it until I was in that situation.

I suspect I would just start getting progressively farther and farther away while they were falling asleep, going in the other room to do stuff etc, maybe while singing with the door open and responding, making some noise, then I would close the door and stay nearby (and tiptoe away very quietly). Then if they call out at any point I guess I would need to go back, and repeat daily until they get used to having me outside the room with the door closed making some noise in the other room while they go to sleep.

I have not read any books or articles on these subjects (including the link), but I have the confidence that I can figure it out with my own good common sense. If I couldn't figure it out, I'd probably start talking to people that I trust for ideas.

I want to be left alone in order to do this! Not a fan of anyone reading some peer-reviewed journal articles and statistics and doing some backseat parenting or throwing around terms like child abuse when discussing how parenting should be done (I can see that a lot of people are wading in without any idea of what they are talking about). Before having kids, I may even have been one of those people (but I doubt it). A lot of ideas sound great on paper. Lots of people with great ideas that worked for them. Good for them. I would appreciate if they would share, but stop short of dogma and the incredible self-centricness in thinking that their way is the only good way.

If the backseat parents really care so much, they should demonstrate their fitness to judge by attracting a mate (this is probably the main hurdle for them), gather the resources and space to procreate, test their theories, and feel free to share afterwards once they have proven out their ideas! They will quickly find that their dogma and lack of basis in grounded reality makes them horrible parents, maybe they won't even have the self-consciousness to realize this. But that's opening a whole different discussion.

When you do something yourself, that's when you get to decide and test out your theories, and not before.