Are you still having this problem?
The downloadable font: rejected by sanitizer error suggest that the Font Awesome font file is corrupt which would explain why the icons aren't displayed.
The corrupt file could just be the one in your cache which is why I suggested purging it. But when one purges the cache the rebuilt cache is identified by the current timestamp (it's using the Unix epoch format, for example 1651313464). On the server the cached files would be in folder moodledata/localcache/theme/<timestamp>. So if you purge caches 1) the timestamp name should change for that folder, and 2) the timestamp should change in the links to files such as the Font Awesome .woff2 file. But after you purged caches your timestamps showed a value from before that purge. I'm not sure what might cause this.
Some things you can try:
1. Delete the cache, localcache and muc folders from Moodledata. To be safe you could move these somewhere else like /tmp but Moodle should automatically rebuild everything in these folders.
2. Purge all caches again, and then run the SQL query:
SELECT name, value, FROM_UNIXTIME(value) FROM mdl_config WHERE name = 'themerev';
This should show you the Unix timestamp for the current theme cache. The last column shows the timestamp as date/time and this should be the time when you purged the cache. The same timestamp should be in moodledata/localcache/theme/<timestamp>.
3. Check the hash of the Font Awesome file lib/fonts/fontawesome-webfont.woff2 with sha1sum. For example if your Moodle source code folder is /var/www/html/moodle run:
cd /var/www/html/moodle
sha1sum lib/fonts/fontawesome-webfont.woff2
This hash should be d6f48cba7d076fb6f2fd6ba993a75b9dc1ecbf0c. If it's different then there's something wrong with this file and possibly other binary files. A problem that used to occur in the days of
FTP was that files were transferred using ASCII (text) mode so PHP files transferred okay but binary files were corrupted.