iMovie hangs with spinning wheel

I'm trying to create a movie from some GoPro Hero 10 clips. Clips are correctly imported - I can play them with QuickTime with no issue.

But I can't do anything with iMovie; thumbnails are blue, most operations (preview, delete from library...) constantly hang with the spinning wheel. It is totally unusable.

I have tried all workarounds I have found; resetting preferences, creating a new library, resetting, NVRAM, reinstalling iMovie... to no success.

I have finally reinstalled the whole OS (macOS Monterey), applied patches and installed latest version of iMovie. Still no success, again the spinning wheel. iMovie show as "not responding":

This is extremely annoying. Any hint?

Many thanks!

Hardware: MacBook Pro (13-inch, 2020, Four Thunderbolt 3 ports), 2 GHz Quad Core i5, 16 GB memory

OS: macOS Monterey 12.4 (21F79)

iMovie version 10.3.3 Build 405303