So what I'm suggesting is that you announce ahead of time and let people who were considering a change go ahead.
Then when that deadline is reached, you offer those buyout packages to the low performers or others you don't want, until you reach your target.
Is the idea that the company tells the affected employees, "Hey, we're planning on firing you in 6 months. We'll pay you severance then, unless you've already quit and found a new job. Or, if one of your peers-who-we're-not-planning-to-fire finds a new job, we'll keep you."
If I'm the company, that doesn't seem very attractive to me -- I've told the employee something that's going to taint the business relationship and make the next six months very awkward.
If the idea is that the company says generally "We need to reduce headcount by 14% in 6 months", I'm not convinced this changes the stress factor much. During those 6 months, a bunch of employees will be seeking assurances that _they_ won't be in line to be fired. This will create a bunch of tension among colleagues. In 6 months, a whole bunch of people are still going to get fired. They'll have had 6 months in which they could perhaps have saved more money, but they're still getting fired. The trauma is still going to be significant.
However, I think a lot of people really struggle with uncertainty. During these six months, especially in a large corporate environment, there would be a lot of horse-trading. Employees will seek assurances they won't be fired. They may avoid projects or people they think are likely to get cut.
At the same time, the business likely has an idea of where they want to go. The "in" managers will navigate their preferred people to safe projects. But there isn't room in the boat for all of their employees -- after all, the business has announced the target for layoffs.
This was my experience when I was at Microsoft during their horribly ill-conceived layoffs in 2009. They basically announced that there would be 3 rounds of layoffs tallying up to 5,000 people over the next several months. It was... incredibly demoralizing.
I still remember one fellow on my team who got fired in the 2nd or 3rd round. He took it poorly (understandably!) and then ripped into the people who didn't get fired (also understandable, but still really shitty).
I don't really know that the advance warning helped him. I think he knew he was likely to get fired when layoffs were announced. Being a dead man walking... not very good for anyone, really.
Uncertainty is important in general but right here right now I’ll take it.
(1) you're fired immediately, with 3 months severance pay
(2) 6 months notice, at the end, you're fired with no severance pay
you'd prefer the second choice?
That's reasonable!
The uncertainty I was describing in my comment applied not only to the fired employees, but to the ones who were being kept. From the company's perspective, there's value in providing clarity to those employees. That's why they'd rather pay 3 months severance (and get no labour from the employee) vs paying 6 months notice (and, theoretically, getting 6 months of labour from the employee).
You may not get the number of volunteers you need, so you still have to do layoffs. Except now, more people have been stressing about it for a longer period of time.
The "low performers" who will have a hard time finding a new job elsewhere are unlikely to voluntarily leave. So you offer a buyout package to derisk the decision for them. But then the "high performers" who you'd rather retain might decide that yeah, it's easy to get a new job, so they'll take a sack of cash and go do something new.