Your intention may be genuine. I can't judge whether you are truly interested in the benefit of both parties. However, what you're asking to be able to do is effectively take control over something that personally benefits employees and not the company. That is not something many employees are going to agree with. Specifically the ones willing to leave will be the ones most intent on guarding that information.
To that end, your question is indistinguishable from:
How do I get my employees to give up something that benefits them, instead making it benefit the company?
Very simply put, you can't. Not unless employees willing choose to do so, and the smart ones tend not to want to do so. Your employees are acting in their own benefit, not that of the company.
The ability to look for a job without having to already start burning the bridge on your current job is very much to the employee's benefit. And the company has a similar benefit, which you are literally trying to rely on right now:
I just want to know how aggressive I need to be about contingency planning and for which teams and what kind of roles I might need to start planning to get (if I even can in this market)
Just like your employees want to see what's out there for them, so do you. You both have the ability to shop around without needing to inform the other party that you are shopping around.
One of the senior devs instant quit over my company announcing that it will be office centric again.
Your company made a decision that has a massive impact on employees' personal lives and work/life balance, and did so without consulting with the employees.
I'm hard-pressed to call this anything but blindingly obvious that people are going to leave without your company knowing so in advance.
Management dealt with the concerns about that very badly and lectured employees about negativity.
When your company does receive feedback, albeit in a reason for quitting, what do they do? They turn around and blame the staff for "negativity", which loosely translates as "things the company doesn't like".
This sounds like a company that rather blames others than look inwardly, and it doesn't sound like a company with a healthy relationship with its staff to me.
Imagine if your boy/girlfriend dumps you, specifying they're doing so because of XYZ. Instead of working on XYZ, you tell all your future dates that they better not be "negative" like your ex. Does that sound like a healthy approach?
I just want to know how aggressive I need to be about contingency planning and for which teams and what kind of roles I might need to start planning to get (if I even can in this market).
This is precisely what a notice period is for. It is determined to be a reasonable amount of time (for both parties), where the active party (quitting employee, firing company) allots time to the passive party (fired employee, company) in order to prepare for the end of the employee's employment.
If you're unable to prepare in this timespan, you've got bigger fish to fry than just one person leaving. That's a disaster waiting to happen. As staff availability is then a liability, your company will need to consider upping either retainment or acquisition of staff, preferably both.
As the proverb goes, you catch a lot more flies with honey than with vinegar. This proverb quintessentially sums up the cause of, and solution to, staffing issues.
One of the great ways to improve staff retainment is by increasing the quality of life of the staff. At the same time, giving your employees a better quality of life also increases the likelihood someone will want to work at your company.
Fortunately, you find yourself at the crossroads of a major quality of life decision for your employees: how to approach remote work post-pandemic. I can't tell you what choice your company needs to make as I don't have all the contextual details, but there is a great opportunity to fix things here.