Ever since I started using Linux (RedHat before becoming a commercial distribution and then Slackware for a total of about 20 years) I have always installed the antivirus, including ClamAV in /opt with <prefix>.
Configuration files have always been in /opt/clamav/etc
Now they will be in /opt/clamav/etc/clamav whatever ...
I will manage to modify the startup scripts (not systemd, it doesn't exist in Slackware) but from my point of view it doesn't help me at all!