One fair idea is to reinstall Catalina in macOS Recovery; some
users resolved issues in later Safari 14 and last version Catalina
10.15.6 by so doing. Other attempts to revert Safari returned
the browser to earlier Safari 13.1.2; some said that was OK.
As for ad-blockers, certain better web browsers include one.
Regional preferences have followings. A cure should be, in
the resolute use of macOS Recovery; to fix new Safari bits.
Of several articles with methods to reinstall macOS, a few
are better; like travel map plans. The route is a destination.
(As to cited Support article ~ that may not go far enough.)
A few tried reverting to older Safari 13.1.2 by reinstalling
Catalina via Recovery; that works if details get straight.
"Is there a way to revert back from Safari 14?" macrumors
A 'Terminal command' was also used by some, for better effect.
Once 'Big Sur' is released there may be a new patch or version
for Safari to work better in the older macOS Catalina & Mojave.
Another idea to Revert to previous macOS version in Support.
However that may be a bridge too far; past the earlier bridge.
Or see/review this official guide 'Reinstall macOS - Apple Support'
{This one does not include many links to optional processes.}
As a guide, it does show the general Table of Contents, unto itself.
• How to reinstall macOS from macOS Recovery - Apple Support
https://support.apple.com/en-us/HT204904
Other thorough instruction ^ offers fuller range of recovery shortcuts.
Less shortcut choices, could push: 'Reinstall macOS - Apple Support'
Sorry for the problematic issues made available by Safari 14 being
made available; from the Big Sur/Safari upgrade file, to Catalina. I'd
not even considered the new Safari14 for my Mojave 10.14.6.
(Other user issues to avoid: involve premature upgrade in iPhone to
the later iOS 14; while other hardware cannot be upgraded, to match
lacking companion upgrade in Mac to Big Sur, or last steps Catalina.)
Take care & good luck!🌻🐝
