Fire them anyway. Is there an actual law? If so, ask the Congress to change it. They'll do it, in this atmosphere. All it takes is a majority, if you're the Prez.
Before that happens, if you cut departments, it doesn't apply, does it?
How worthwhile is it to get the skunks out of the yard? Target them and buy them out. Cheap at twice the price. If they won't go, reassign them somewhere boring where nothing important is happening, where they can be watched.
If it's Trump, it could easily happen. The mood of the country is wide open to this now.
Most are protected under Civil Service regulations.
Bear in mind that whenever cuts loom (even minor one’s) agencies act swiftly to shut down things like national parks, threaten delays in social security checks, stop processing applications, etc. They will claim that resource constraints prevent them from performing vital services and they will get their friends in the media to launch a campaign of disinformation to rile up the people. Agencies are well-schooled in the art of protecting themselves.
Yes, there is such a thing a buyouts but generally that only applies to those with a lot of years of service. And even then, they may not leave.
The point is, it won’t be an easy thing to accomplish and it will take a lot of thoughtful planning.