OK, it works (like expected).
I'll use the following file as conf. file for logrotate:
/home/user/.config/joplin-desktop/log*.txt {
weekly
rotate 4
missingok
notifempty
compress
copytruncate
}
This will rotate weekly (and compress) every log file found in /home/user/.config/joplin-desktop/ and keep 4 old files.
Hint: one does not need to restart any service after putting this file into place (/etc/logrotate.d) since logrotate is not a daemon but started via crontab/systemd timer.