This is the way, I implemented automatic updates of spamassasin rules for MailPlus Server on my Synology DS920+, running on DSM 7.0.
MY Finding: DSM 7.0 is using rspamd instead of spamassasin, this means, some older scripts, created for DSM 6 or earlier might not work anymore.
This script and description come with no warranty, I cannot held responsible, if something breaks on you machine!