Meanwhile, major manufacturers seem to have rock-solid, practically trouble free BMS performance.
Which specific manufacturers?
I’d love to hear from anyone who’s torn down a “big brand” battery or worked behind the scenes.
Which specific "big brand"s?
Without knowing what exactly you are comparing to, it's tough to give you useful info.
But, in general, any company designing a product for a specific use, especially one that is hazardous if not designed and built correctly, will have to have (or hire out for) competent engineers to design it, competent QA people to design the verification and testing processes, and be willing to invest the money it will take to create something that does the job safely and as well as possible for as long as possible so they will not have to do any warranty repairs.
There are certainly a lot of companies making batteries that definitely do NOT do any of those things, but there are some that do--most likely the ones that can be sued if their stuff causes a problem or kills someone.
Most of the places that manufacture parts / boards / modules / etc will custom make a version of them for a company that is buying enough of them. But...many of those will not be usable outside their "ecosystem", as they may have custom firmware requiring some other part of the system to connect to, communicate with, etc. Some will be "generic" and work just like any other customer-purchasable similar part, just differently (perhaps better, perhaps not).
If you really want to know what some specific company is using that is different than what you can buy, you'd probably have to buy one of that company's units, then take it apart to see what's in there, and if it has any markings that indicate who made the parts for them (if they didn't make them themselves, which they might).
If you have a specific thing you need your system to do for you, then I'd recommend making yourself a complete list of features and requirements, and a separate list of "would be nices", each list ordered by most required down to least, prioritized so you can decide if something that has all but one of them is still acceptable or if you really have to have them all, etc. Anything you leave off the list that you require means anything you find that fits all the rest but not that is a waste of time...so be thorough.
Posting that list may help us help you find something that does what you want (if it exists).
