With absolutely no recent input from me, the problem that I've had since December 2020, with Safari Bookmark Beachballs (Both Safari and Safari Technology Preview), appears to have disappeared on it's own sometime overnight. At least with the latest Safari Technology Preview Release 125 (Safari 14.2, WebKit 16612.1.15.1.12), that is. The SBB still persists with Safari 14.1.1.
The continuous SBB, while using both Safari and Safari Technology Preview, started about the same time that I installed Big Sur on two desktops, and a laptop. All three are running 11.4.
The SBB problem never presented itself on my late 2013, 27 inch iMac, running 10.15.7. Or did it crop up on four iPads, three with the latest OS.
Every Apple device except for the oldest iPad, had the the exact same number of bookmarks.
I jumped through countless hoops trying everything that was recommended. But it was an exercise in futility.
I normally run Safari Technology Preview on the three desktops and one laptop. Unfortunately, I just noticed that 2013 Mac will no longer update Safari Technology Preview. Oh well, at least Safari on that Mac, does not suffer from SBB.
I first noticed my good fortune of no SBB this morning, when I fired up Safari Technology Preview on the laptop, and started surfing. It didn't take long before I noticed just how much quicker the browser was, with no SBBs. I sure hope the SBBS stay away.
I haven't tried the third iMac running 11.4 yet, but I think the most recent Safari Technology Preview, has put a stop to the dreaded SBBs.
On a side note, Activity Monitor always showed an increasing tally of 'Hang's, whenever there were SBBs.
Yet, over the past 6 months, EtreCheckPro on any of the affected computers, never showed any problems with either Safari or Safari Technology Preview. Even though the dock, and perhaps the finder would freeze, until the SBBs ceased.
For now, I'm elated. Just as long as the SBBs stay away.
As for what transpired overnight, perhaps an Apple employee finally flicked that switch, under that flashing red light.
https://discussions.apple.com/thread/252114600
https://discussions.apple.com/thread/252162522
https://discussions.apple.com/thread/252382776
https://discussions.apple.com/thread/252396123