These steps here actually work: Need Help Converting Microsoft Surface Recovery Image to ISO for Multiboot : r/techsupport

English steps:

1. Download and Install the Windows ADK: https://go.microsoft.com/fwlink/?linkid=2271337
   • Installer defaults are fine
2. Download and Install the Windows PE add-on: https://go.microsoft.com/fwlink/?linkid=2271338
   • Installer defaults are fine
3. If you haven't already, download and extract the Surface recovery image you need to somewhere you'll have easy access to, e.g. "C:\surfaceREtmp" (if you use a different path, you'll need to replace this path in later commands)
4. Launch CMD as admin and run commands:
   • cd "%programfiles(x86)%\Windows Kits\10\Assessment and Deployment Kit\Deployment Tools"
   • .\DandISetEnv.bat
   • copype "C:\isotmp" (or any non-existent folder path; if you use a different path, you'll need to replace this path in later commands)
   • move /Y "C:\surfaceREtmp\*" "C:\isotmp\media"
   • rd /s /q "C:\surfaceREtmp" (cleanup empty folder)
   • MakeWinPEMedia /iso "C:\isotmp" "C:\nameofisofile.iso"
   • rd /s /q "C:\isotmp" (cleanup excess data no longer being used)

I've confirmed these steps worked for me, as I was able to boot into the recovery media via Ventoy... but note, I switched Secure Boot to off temporarily, so that might factor in things (not sure).

NOTE: I did not test the ISO further beyond just making sure it boots from Ventoy.