As you are probably already aware, with the upgrade to BigSur on Macs, at least three versions of LibreOffice Writer are showing blurry fonts in the editing mode. I've tried editing with version 7.0.3.1 and with 6.4.7.2 and with 6.4.4.2. Blurry fonts with all three.
*** This bug has been marked as a duplicate of bug 138122 ***