@gpmac You're not the only one, another thread here: https://developer.apple.com/forums/thread/650365?answerId=698512022#698512022.
J7mbo
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You might find that over several hours the bar increases. If you check activity monitor for disk io, you'll likely see there's a lot of ever-increasing reads / writes for installd. If so, then it's just taking a long time to install. Keep your Mac on and be patient.