Re: GarageBand freezing on 2019 iMac with 3.6 GHz Core i3 and 8GB RAM
Wondering whether your iMac has plenty of memory / storage space free for the work you are doing with  Garageband. Eg: A Mac normally requires around 50 Gb free in which to operate smoothly  and do all the tasks we require of it. Some memory-intensive apps / large projects may work best with even more free space. 
(NB: Any free storage space you may have available in iCloud is quite separate from storage on Mac)
Eg: For MacOS 15 : See used and available storage space on your Mac - Apple Support
Note: If space is short on you  Mac, it is always wise to back-up Mac data, before making space available.
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If memory / storage space is not-a-problem:
If you haven't tried it: Try restarting you iMac: (deletes temporary files, re-establishhes connections, may update recent stting changes, and often restores order if something is not working as expected.
Eg: Apple menu > Restart ... log back in  after Mac restarts, and see if there's any improvement.
You may also want to check if there's a Garageband update available for your iMac, in the App Store:
Open the App Store > Click updates in the left sidebar to view any updates available  for your system.
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If needed: More info. / support / contact options etc. 
Garageband Support website : GarageBand – Official Apple Support
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       Resource: GarageBand User Guide for Mac: