How to create /etc/certificates/*.{cert,chain,concat,key}.pem files without reboot?
1. Also import the .fullchain
Beside the .cer file also import the corresponding .fullchain file into the keychain, like
$ sudo /usr/bin/security -v import /Users/user/letsencrypt/fullchain.cer -k &quot;/Library/Keychains/System.keychain&quot;

2. Unload and load UserEventAgent-System
$ sudo /bin/launchctl unload /System/Library/LaunchDaemons/com.apple.UserEventAgent-System.plist; sudo /bin/launchctl load /System/Library/LaunchDaemons/com.apple.UserEventAgent-System.plist

3. Wait
About 1-2 minutes for the 4 certificate files to correctly appear in /etc/certificates/