iMovie 10.2.5 requires Metal API support. You need either:
Comprehensive Guide to iMovie 10.2.5 DMG: Features, Requirements, and Updates imovie 1025 dmg upd
codesign -dv --verbose=4 /Volumes/iMovie\ 10.2.5/iMovie.app iMovie 10
Updated interface elements to match the aesthetic of macOS Big Sur. Performance Improvements: imovie 1025 dmg upd
That will give you a for your macOS, wrapped in Apple’s safe installer — no shady .dmg needed.