I feel like you may be trying to put a square peg into a round hole.
Your problem is that you just laid off a ton of people, including your lead developer's best friend (at the company), and you want the rest of the people staying to "feel safe". You are trying to solve that problem by throwing money at it, as if making $100 now is going to make me feel better when I get fired in 3 months anyway.
The thing about job security is that it is job security. If you pay me $100 more now, or $1000 more, or $10000 more, then you fire me in a month, then in a month from now I'll have zero income and have to find another job. Even if it is easy to find another job (and that isn't always the case, even when you think it is), it's still a hassle, and you may wind up making less money than you were before because you can't negotiate a salary when you have none to compare against.
This is not a problem that gets solved with money. Money is, of course, nice, but the core issue is that people will start looking for a new job as soon as they lose their feeling of job security, to minimize the amount of time they are without salary, and not a moment earlier or later. So even if you give this dude a substantial raise to keep him on, the fact of the matter remains that you just fired 10% of your company which screams some kind of managerial problem, financial insolvency, or some other such issue, and if it were me I'd want to jump off that ship ASAP while there are still lifeboats available, so to speak.
This begs the obvious question: If you give a person a "retention raise", why would that person not have faith in their job security? And the answer is that companies do shady shit all the time. I had a job a while back where I got a raise, and a promotion, and a glowing review from my boss who promoted me, and within 6 weeks I was terminated (the company was insolvent). Just because you give the person a raise doesn't mean (to me at least) that you're not going to do something shady and turn around and turf him the next day, and would not change my opinion regarding if I am/should be job hunting elsewhere.
So, the real question is, if throwing money at the problem isn't a solution, what is the solution?
Well, I never said that throwing money at the problem doesn't help, only that it's not a full solution. If you pay this guy a ridiculous rate that no other company would even think of matching, then he's less likely to quit, simply because he'd have to take a pay cut if he were to quit; absent other considerations it may make him less likely to make a move, because making such a move would cost him money. This strategy only works if he would be quitting for reasons other than job security, such as a retaliatory move for the aforementioned firing of his friend; as I said, there is no reason a company wouldn't do something shady like firing someone immediately after giving them a massive raise, and there is no reason that shouldn't be in the back of this employee's mind, particularly if he perceives the company to be insolvent and you're offering to make that insolvency problem worse by giving away more of the company's money.
However, be aware that if you intend to try to solve this problem with money, it has to be a lot of money. You're going to want to go above the pay cap of any local competitor by at least 15-20%. To put this into perspective, let's say you're a small company and you are in a region where Google has a regional office. Your small company, being a small company, will pay a good developer $150k. Google will pay a good developer $250k, but Google's interview is way harder to pass than yours, because they're Google. Your goal is to make this person not want to even attempt Google's interview, because if they attempt Google's interview then there is a chance they will pass (especially since you value this guy so much, the chances he's good enough for Google are high), and they will leave. So, in order to have that, you're going to have to pay him more than Google would, if he tried and passed Google's interview. Which would put his salary probably around $300k in order to make him not want to leave. Can you afford to give him such a salary increase?
But again, that doesn't solve the real problem, which is that if a company fires 10% of their workforce, then they are seen as insolvent. Companies don't just fire 10% of their workforce, including an entire department, "just because". If I was such an employee, my first thought would be either that management is cleaning house, or that management is trying to cut costs, both of which boil down to me being very concerned about my job security. You're going to have to be creative about solving this one; the goal is that you want your employees to not feel like they need an exit plan from your company on an immediate basis, which is how they feel right now. A possible solution to this would be some kind of written documentation that no other employees will be fired for a certain amount of time, with some actionable punitive damages to the company on the condition that someone was fired this way. Something like this:
To all employees:
As you may know, ABC Software Inc. just went through a large round of layoffs. Management would like to assure you that the company is not insolvent and we are continuing operations as normal. In order to make you all feel secure in remaining with ABC Software Inc. for the future to come, in addition to this letter promising that our current layoff round is complete, we will be paying $20,000 bonus severance (in addition to any other severance promised) to anyone terminated without cause from ABC Software Inc. until December 31, 2021, who was fully employed (not on probation or PIP) as of August 18, 2021.
Thank you all for your continued service through these trying times.
If I was to receive this notice, it would make me feel that the company is really not going to fire me, at least not this year, because it would be prohibitively costly for them to do so, and I wouldn't need to find another job. Of course, this does make it difficult to fire anyone for the rest of the year without a significant reason for doing so, but them's the breaks of breaking your employees' trust; you have to take the burden of repaying those costs.
There may be other solutions as well (this is just one I can think of), so be creative!