Thank you for trying those Troubleshooting checklist things first. That helps narrow it down.
I have a few ideas, but I’ll admit they mostly of the “I wonder if…” variety. I’ll leave it up to you whether they sound worth trying or not. [In case it becomes helpful, this thread has ideas about what sorts of things cause it to pop up. How to find out what Anki is "Processing..."? - #6 by dae]
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If the cards are all from the same note type, is it possible that the templates are running scripts that are buggy or taking extra time to finish?
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Have you tried running Anki from a terminal window? It’s not a crash, but if there are errors being thrown, they might show up there.
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Have you tried exporting your collection as apkg or colpkg, and importing it into a fresh profile?