At risk of outing myself to anyone who has ever read anything I have written on the matter before...
Separation anxiety is normal for puppies, it is their default setting for very good reasons and it is not something we've selectively bred out of dogs.
This means we should avoid them experiencing it, at pretty much all cost (barring life or death shit really).
Turning normal puppy inability to handle being left alone into a long term and really difficult to fix adult abnormal behaviour can be done by:
Leaving a puppy too soon
Leaving them for too long
Failing to provide the security and build the confidence that they need
Subjecting them to long term constant stress, lack of sleep (see all the 'make your puppy sleep by themselves, ignore crying' people for this one).
It can also be triggered by one off unpleasant events that occur whilst alone or in some way linked to being alone.
Puppy confidence comes and goes, in the first year we have to be REALLY careful to avoid unpleasant experiences and that might be them being alone too long, being upset or frustrated, or something actively nasty happening (house fire, someone bashing on a door for ages, sudden scary window cleaner... ). What a puppy can cope with this week is a very poor indicator of what they'll manage next week.
Are there some puppies who will tolerate it and be absolutely fine, gradually getting used to longer and longer periods - of course there are. Are there some who will cry a couple of times and then appear fine, absolutely.
But you cannot guarantee a puppy will do that, by simply looking at the puppy, there is no way to tell which will cope and which will not.
Accepting that what we're asking is hugely unreasonable and abnormal, and taking it carefully is the sensible plan. If we are wrong, then oh dear, it's been an intensive year raising the puppy, ensuring those foundations are in place, and we've gone slower than perhaps we could have.
If however, we assume they'll be fine, and we are wrong - oh dear we've got a dog who self harms, trashes crates, panics and pisses everywhere, smashes out of windows, rips sofas/blinds, howls... needs months or YEARS of behaviour modifcation to get over it, or needs to be rehomed. Or needs to be euthanised.
It is a year to 18 months of your time, out of the 12+ years you'll have with a dog. If that is too much to ask, don't get a dog. Just don't.