Good to hear Nextclouds log-rotation is working.
For the apache log checkout logrotate. You’ll find a lot HowTos if you search google for logrotate howto.
You’ve got plenty of 500 errors in your apache logfile. I’d suggest to try to eliminate those - that will reduce the filesize as well 
I don’t know much about apache though - so I can’t be a big help with that. Sorry.