Sounds like what you need is to enable/configure log rotate. I haven’t had the need to do this myself. Don’t think my log gets that massive.
I’m not where I can provide details or do any testing. Just search for log rotate on Raspbian Buster. I think I remember seeing a recent change where the it’s controls by systemd now instead of a cron job.
73,
Aaron – N3MBH
OpenRepeater is offered free of charge. Find out how you can support us.