With macOS 10.14.6+, Final Cut Pro X 10.4.6+ automatically uses the GPU in the Breakaway Box, Breakaway Puck, or DuoModo eGPU Module is used to accelerate Final Cut Pro X.
Connect one 4k monitor to the DP port.
Connect one 4k monitor to HDMI port.
Connect two 4k monitors a Sonnet Thunderbolt 3 to Dual DP or Dual HDMI adapter connected to the Thunderbolt 3 port.
macOS 14 (Sonoma) and macOS 15 (Sequoia) will crash when an eGPU is hot disconnected. It will work fine if you connect or disconnect when the computer is shut down. Sonnet has filed a bug report with Apple.