Success! (Eventually). It took about 1 hour and 30 minutes to run the update, plus a few minutes preparation to update kexts and clover bootloader.
Updated kexts, used Hackintool to update clover to 5107, then rebooted.
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Overview of the installation process (sharing because it seemed a bit... weird).
Screen change #1: 5m. White background, gray apple; lots of thin, horizontal lines in the upper half of the screen (photo attached for reference. Hadn't seen this before, so was worried, but it wasn't a freeze).
Screen change #2: 3m. Completely black screen. Could easily have been mistaken as frozen (cough)
Screen change #3: 4m. White background, gray apple; lots of thin, horizontal lines in the upper half of the screen.
Screen change #4: 10m. Completely black screen.
Screen change #5: 2m. Regular boot screen (Yay! Right?)
Screen change #6: 18m. Completely black screen.
Screen change #7: 35m...? Completely black screen. Finally gave up and rebooted.
(Regular reboot went to black screen. Uh-oh).
Used F12 to select UEFI P0 (just in case)
Screen change #8: Wonky Clover Bootloader screen, with broken icons. Not promising.
Screen change #9: 10 m. Black background, white apple with progress bar ('10 minutes remaining')
It worked! Booted to desktop, version checks out at 10.15.4. Hurray!
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on edit:
[Minor issue: the update broke the icons/theme for clover bootloader]