WDYT
I think this particular style of syntax is running into a number of issues related to expectations about the function being called taking one parameter (or fancy syntax to get around that), or expectations around how the resulting value will get used (or fancy syntax to get around that).
I don't know how far you read into some of the other threads you linked to, but I'm partial to this little idea I threw around over here - i.e. we just put a "..." inside a callback instead of a normal body, to signify that we want the rest of the outer block to be treated as part of the callback. It doesn't look quite as pretty as "<=<", but it also deals with all of these edge cases very neatly and it's, IMO, easier to get a grasp on what it's doing.
Another option would be to request a backwards pipeline operator, that's intended to be used for the purposes of flattening a nested chunk of callbacks - something I once brought up here - which was an interesting discussion.
In short, I think the problem of handling nested callbacks is an interesting one to solve, but I believe the solution for it has to be very simple - the problem just isn't big enough to warrant anything else.